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/GPU3D.cpp

@@ -22,6 +22,7 @@
 #include "NDS.h"
 #include "GPU.h"
 #include "FIFO.h"
+#include "GPU3D_Soft.h"
 #include "Platform.h"
 
 namespace melonDS
@@ -139,7 +140,9 @@ const u8 CmdNumParams[256] =
 
 void MatrixLoadIdentity(s32* m);
 
-GPU3D::GPU3D(melonDS::NDS& nds) noexcept : NDS(nds)
+GPU3D::GPU3D(melonDS::NDS& nds, std::unique_ptr<Renderer3D>&& renderer) noexcept :
+    NDS(nds),
+    CurrentRenderer(renderer ? std::move(renderer) : std::make_unique<SoftRenderer>(nds.GPU))
 {
 }
 
@@ -2336,6 +2339,12 @@ void GPU3D::RestartFrame() noexcept
     CurrentRenderer->RestartFrame();
 }
 
+void GPU3D::Stop() noexcept
+{
+    if (CurrentRenderer)
+        CurrentRenderer->Stop();
+}
+
 
 bool YSort(Polygon* a, Polygon* b)
 {
@@ -2888,6 +2897,12 @@ void GPU3D::Write32(u32 addr, u32 val) noexcept
     Log(LogLevel::Debug, "unknown GPU3D write32 %08X %08X\n", addr, val);
 }
 
+void GPU3D::Blit() noexcept
+{
+    if (CurrentRenderer)
+        CurrentRenderer->Blit();
+}
+
 Renderer3D::Renderer3D(bool Accelerated)
 : Accelerated(Accelerated)
 { }