Refactor DSi_NAND (#1844)

* Refactor diskio's contents

- Change ff_disk_read_cb/write_cb into a std::function instead of a raw pointer
- Add const specifiers as needed

* Refactor DSi_NAND to manage the file system's mounted lifetime with RAII

* Split NANDMount into NANDMount and NANDImage

- NANDImage is used for information about the NAND that doesn't require decryption or filesystem access
- NANDMount is used to actually access the file system
- Both classes manage their respective resources (the NAND file handle and the NAND's mount) with RAII
- Also split the file loading into another function that I will remove in a later PR

* Make NANDMount immovable

* Remove NAND-loading code that I had sectioned off into a function

- Incomplete copypasta
- I must have gotten distracted

* Tidy up NANDImage's initialization

- Don't unmount the disk image if the constructor fails (that's NANDMount's job now)
- Only assign CurFile if the constructor succeeds

* Add some const-correctness

* Move DSi NAND initialization to the frontend

- The NANDImage is now installed via a unique_ptr in DSi

* Remove Platform::DSi_NANDPath

- Not Config::DSiNANDPath; that can still be configured as usual
- The core no longer needs to care

src/FATIO.cpp

@@ -0,0 +1,162 @@
+/*
+    Copyright 2016-2023 melonDS team
+
+    This file is part of melonDS.
+
+    melonDS is free software: you can redistribute it and/or modify it under
+    the terms of the GNU General Public License as published by the Free
+    Software Foundation, either version 3 of the License, or (at your option)
+    any later version.
+
+    melonDS is distributed in the hope that it will be useful, but WITHOUT ANY
+    WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+    FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License along
+    with melonDS. If not, see http://www.gnu.org/licenses/.
+*/
+
+#include "FATIO.h"
+#include "fatfs/ff.h"
+#include "fatfs/diskio.h"
+
+static ff_disk_read_cb ReadCb;
+static ff_disk_write_cb WriteCb;
+static LBA_t SectorCount;
+static DSTATUS Status = STA_NOINIT | STA_NODISK;
+
+
+void ff_disk_open(const ff_disk_read_cb& readcb, const ff_disk_write_cb& writecb, LBA_t seccnt)
+{
+    if (!readcb) return;
+
+    ReadCb = readcb;
+    WriteCb = writecb;
+    SectorCount = seccnt;
+
+    Status &= ~STA_NODISK;
+    if (!writecb) Status |= STA_PROTECT;
+    else          Status &= ~STA_PROTECT;
+}
+
+void ff_disk_close()
+{
+    ReadCb = nullptr;
+    WriteCb = nullptr;
+    SectorCount = 0;
+
+    Status &= ~STA_PROTECT;
+    Status |= STA_NODISK;
+}
+
+
+/*-----------------------------------------------------------------------*/
+/* Get Drive Status                                                      */
+/*-----------------------------------------------------------------------*/
+
+DSTATUS disk_status (
+	BYTE pdrv		/* Physical drive nmuber to identify the drive */
+)
+{
+	return Status;
+}
+
+
+
+/*-----------------------------------------------------------------------*/
+/* Inidialize a Drive                                                    */
+/*-----------------------------------------------------------------------*/
+
+DSTATUS disk_initialize (
+	BYTE pdrv				/* Physical drive nmuber to identify the drive */
+)
+{
+    Status &= ~STA_NOINIT;
+	return Status;
+}
+
+
+
+/*-----------------------------------------------------------------------*/
+/* Read Sector(s)                                                        */
+/*-----------------------------------------------------------------------*/
+
+DRESULT disk_read (
+	BYTE pdrv,		/* Physical drive nmuber to identify the drive */
+	BYTE *buff,		/* Data buffer to store read data */
+	LBA_t sector,	/* Start sector in LBA */
+	UINT count		/* Number of sectors to read */
+)
+{
+	if (Status & (STA_NOINIT | STA_NODISK)) return RES_NOTRDY;
+	if (!ReadCb) return RES_ERROR;
+
+	UINT res = ReadCb(buff, sector, count);
+	if (res != count) return RES_ERROR;
+	return RES_OK;
+}
+
+
+
+/*-----------------------------------------------------------------------*/
+/* Write Sector(s)                                                       */
+/*-----------------------------------------------------------------------*/
+
+#if FF_FS_READONLY == 0
+
+DRESULT disk_write (
+	BYTE pdrv,			/* Physical drive nmuber to identify the drive */
+	const BYTE *buff,	/* Data to be written */
+	LBA_t sector,		/* Start sector in LBA */
+	UINT count			/* Number of sectors to write */
+)
+{
+	if (Status & (STA_NOINIT | STA_NODISK)) return RES_NOTRDY;
+	if (Status & STA_PROTECT) return RES_WRPRT;
+	if (!WriteCb) return RES_ERROR;
+
+	UINT res = WriteCb(buff, sector, count);
+	if (res != count) return RES_ERROR;
+	return RES_OK;
+}
+
+#endif
+
+
+/*-----------------------------------------------------------------------*/
+/* Miscellaneous Functions                                               */
+/*-----------------------------------------------------------------------*/
+
+DRESULT disk_ioctl (
+	BYTE pdrv,		/* Physical drive nmuber (0..) */
+	BYTE cmd,		/* Control code */
+	void *buff		/* Buffer to send/receive control data */
+)
+{
+    switch (cmd)
+    {
+    case CTRL_SYNC:
+        // TODO: fflush?
+        return RES_OK;
+
+    case GET_SECTOR_COUNT:
+        *(LBA_t*)buff = SectorCount;
+        return RES_OK;
+
+    case GET_SECTOR_SIZE:
+        *(WORD*)buff = 0x200;
+        return RES_OK;
+
+    case GET_BLOCK_SIZE:
+        *(DWORD*)buff = 1;
+        return RES_OK;
+
+    case CTRL_TRIM:
+        // TODO??
+        return RES_OK;
+    }
+
+	//printf("FatFS: unknown disk_ioctl(%02X, %02X, %p)\n", pdrv, cmd, buff);
+	return RES_PARERR;
+}
+