Clean up the 3D renderer for enhanced flexibility (#1895)

* Give `GPU2D::Unit` a virtual destructor

- Undefined behavior avoided!

* Add an `array2d` alias

* Move various parts of `GPU2D::SoftRenderer` to `constexpr`

- `SoftRenderer::MosaicTable` is now initialized at compile-time
- Most of the `SoftRenderer::Color*` functions are now `constexpr`
- The aforementioned functions are used with a constant value in at least one place, so they'll be at least partially computed at compile-time

* Generalize `GLRenderer::PrepareCaptureFrame`

- Declare it in the base `Renderer3D` class, but make it empty

* Remove unneeded `virtual` specifiers

* Store `Framebuffer`'s memory in `unique_ptr`s

- Reduce the risk of leaks this way

* Clean up how `GLCompositor` is initialized

- Return it as an `std::optional` instead of a `std::unique_ptr`
- Make `GLCompositor` movable
- Replace `GLCompositor`'s plain arrays with `std::array` to simplify moving

* Pass `GPU` to `GLCompositor`'s important functions instead of passing it to the constructor

* Move `GLCompositor` to be a field within `GLRenderer`

- Some methods were moved up and made `virtual`

* Fix some linker errors

* Set the renderer in the frontend

* Remove unneeded `virtual` specifiers

* Remove `RenderSettings` in favor of just exposing the relevant member variables

* Update the frontend to accommodate the core changes

* Add `constexpr` and `const` to places in the interpolator

* Qualify references to `size_t`

* Construct the `optional` directly instead of using `make_optional`

- It makes the Linux build choke
- I think it's because `GLCompositor`'s constructor is `private`

src/GPU.h

@@ -25,10 +25,6 @@
 #include "GPU3D.h"
 #include "NonStupidBitfield.h"
 
-#ifdef OGLRENDERER_ENABLED
-#include "GPU_OpenGL.h"
-#endif
-
 namespace melonDS
 {
 class GPU3D;
@@ -56,33 +52,30 @@ struct VRAMTrackingSet
     NonStupidBitField<Size/VRAMDirtyGranularity> DeriveState(u32* currentMappings, GPU& gpu);
 };
 
-struct RenderSettings
-{
-    bool Soft_Threaded;
-
-    int GL_ScaleFactor;
-    bool GL_BetterPolygons;
-};
-
 class GPU
 {
 public:
-    GPU(melonDS::NDS& nds) noexcept;
+    explicit GPU(melonDS::NDS& nds, std::unique_ptr<Renderer3D>&& renderer3d = nullptr, std::unique_ptr<GPU2D::Renderer2D>&& renderer2d = nullptr) noexcept;
     ~GPU() noexcept;
     void Reset() noexcept;
     void Stop() noexcept;
 
     void DoSavestate(Savestate* file) noexcept;
 
-    [[deprecated("Set the renderer directly instead of using an integer code")]] void InitRenderer(int renderer) noexcept;
-    void DeInitRenderer() noexcept;
-    void ResetRenderer() noexcept;
-
-    void SetRenderSettings(int renderer, RenderSettings& settings) noexcept;
+    /// Sets the active renderer to the renderer given in the provided pointer.
+    /// The pointer is moved-from, so it will be \c nullptr after this method is called.
+    /// If the pointer is \c nullptr, the renderer is reset to the default renderer.
+    void SetRenderer3D(std::unique_ptr<Renderer3D>&& renderer) noexcept;
+    [[nodiscard]] const Renderer3D& GetRenderer3D() const noexcept { return GPU3D.GetCurrentRenderer(); }
+    [[nodiscard]] Renderer3D& GetRenderer3D() noexcept { return GPU3D.GetCurrentRenderer(); }
 
     u8* GetUniqueBankPtr(u32 mask, u32 offset) noexcept;
     const u8* GetUniqueBankPtr(u32 mask, u32 offset) const noexcept;
 
+    void SetRenderer2D(std::unique_ptr<GPU2D::Renderer2D>&& renderer) noexcept { GPU2D_Renderer = std::move(renderer); }
+    [[nodiscard]] const GPU2D::Renderer2D& GetRenderer2D() const noexcept { return *GPU2D_Renderer; }
+    [[nodiscard]] GPU2D::Renderer2D& GetRenderer2D() noexcept { return *GPU2D_Renderer; }
+
     void MapVRAM_AB(u32 bank, u8 cnt) noexcept;
     void MapVRAM_CD(u32 bank, u8 cnt) noexcept;
     void MapVRAM_E(u32 bank, u8 cnt) noexcept;
@@ -578,7 +571,7 @@ public:
     u8* VRAMPtr_BOBJ[0x8] {};
 
     int FrontBuffer = 0;
-    u32* Framebuffer[2][2] {};
+    std::unique_ptr<u32[]> Framebuffer[2][2] {};
 
     GPU2D::Unit GPU2D_A;
     GPU2D::Unit GPU2D_B;
@@ -611,11 +604,6 @@ public:
 
     u8 VRAMFlat_Texture[512*1024] {};
     u8 VRAMFlat_TexPal[128*1024] {};
-
-    int Renderer = 0;
-#ifdef OGLRENDERER_ENABLED
-    std::unique_ptr<GLCompositor> CurGLCompositor = nullptr;
-#endif
 private:
     void ResetVRAMCache() noexcept;
     void AssignFramebuffers() noexcept;