Allow for a more modular renderer backends (#990)

* Draft GPU3D renderer modularization

* Update sources C++ standard to C++17

The top-level `CMakeLists.txt` is already using the C++17 standard.

* Move GLCompositor into class type

Some other misc fixes to push towards better modularity

* Make renderer-implementation types move-only

These types are going to be holding onto handles
of GPU-side resources and shouldn't ever be copied around.

* Fix OSX: Remove 'register' storage class specifier

`register` has been removed in C++17...
But this keyword hasn't done anything in years anyways.

OSX builds consider this "warning" an error and it
stops the whole build.

* Add RestartFrame to Renderer3D interface

* Move Accelerated property to Renderer3D interface

There are points in the code base where we do:
`renderer != 0` to know if we are feeding
an openGL renderer. Rather than that we can instead just have this be
a property of the renderer itself.
With this pattern a renderer can just say how it wants its data to come
in rather than have everyone know that they're talking to an OpenGL
renderer.

* Remove Accelerated flag from GPU

* Move 2D_Soft interface in separate header

Also make the current 2D engine an "owned" unique_ptr.

* Update alignment attribute to standard alignas

Uses standardized `alignas` rather than compiler-specific
attributes.

https://en.cppreference.com/w/cpp/language/alignas

* Fix Clang: alignas specifier

Alignment must be specified before the array to align the entire array.

https://en.cppreference.com/w/cpp/language/alignas

* Converted Renderer3D Accelerated to variable

This flag is checked a lot during scanline rasterization. So rather
than having an expensive vtable-lookup call during mainline rendering
code, it is now a public constant bool type that is written to only once
during Renderer3D initialization.

src/GPU2D_Soft.h

@@ -0,0 +1,79 @@
+/*
+    Copyright 2016-2020 Arisotura
+
+    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/.
+*/
+
+#pragma once
+
+#include "GPU2D.h"
+
+class GPU2D_Soft : public GPU2D
+{
+public:
+    GPU2D_Soft(u32 num);
+    ~GPU2D_Soft() override {}
+
+    void DrawScanline(u32 line) override;
+    void DrawSprites(u32 line) override;
+    void VBlankEnd() override;
+
+protected:
+    void MosaicXSizeChanged() override;
+
+private:
+
+    alignas(8) u32 BGOBJLine[256*3];
+    u32* _3DLine;
+
+    alignas(8) u32 OBJLine[256];
+    alignas(8) u8 OBJIndex[256];
+
+    u32 NumSprites;
+
+    u8 MosaicTable[16][256];
+    u8* CurBGXMosaicTable;
+    u8* CurOBJXMosaicTable;
+
+    u32 ColorBlend4(u32 val1, u32 val2, u32 eva, u32 evb);
+    u32 ColorBlend5(u32 val1, u32 val2);
+    u32 ColorBrightnessUp(u32 val, u32 factor);
+    u32 ColorBrightnessDown(u32 val, u32 factor);
+    u32 ColorComposite(int i, u32 val1, u32 val2);
+
+    template<u32 bgmode> void DrawScanlineBGMode(u32 line);
+    void DrawScanlineBGMode6(u32 line);
+    void DrawScanlineBGMode7(u32 line);
+    void DrawScanline_BGOBJ(u32 line);
+
+    static void DrawPixel_Normal(u32* dst, u16 color, u32 flag);
+    static void DrawPixel_Accel(u32* dst, u16 color, u32 flag);
+
+    typedef void (*DrawPixel)(u32* dst, u16 color, u32 flag);
+
+    void DrawBG_3D();
+    template<bool mosaic, DrawPixel drawPixel> void DrawBG_Text(u32 line, u32 bgnum);
+    template<bool mosaic, DrawPixel drawPixel> void DrawBG_Affine(u32 line, u32 bgnum);
+    template<bool mosaic, DrawPixel drawPixel> void DrawBG_Extended(u32 line, u32 bgnum);
+    template<bool mosaic, DrawPixel drawPixel> void DrawBG_Large(u32 line);
+
+    void ApplySpriteMosaicX();
+    template<DrawPixel drawPixel>
+    void InterleaveSprites(u32 prio);
+    template<bool window> void DrawSprite_Rotscale(u32 num, u32 boundwidth, u32 boundheight, u32 width, u32 height, s32 xpos, s32 ypos);
+    template<bool window> void DrawSprite_Normal(u32 num, u32 width, u32 height, s32 xpos, s32 ypos);
+
+    void DoCapture(u32 line, u32 width);
+};