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_Soft.h

@@ -29,16 +29,17 @@ namespace melonDS
 class SoftRenderer : public Renderer3D
 {
 public:
-    SoftRenderer(melonDS::GPU& gpu) noexcept;
-    virtual ~SoftRenderer() override;
-    virtual void Reset() override;
+    SoftRenderer(melonDS::GPU& gpu, bool threaded = false) noexcept;
+    ~SoftRenderer() override;
+    void Reset() override;
 
-    virtual void SetRenderSettings(const RenderSettings& settings) noexcept override;
+    void SetThreaded(bool threaded) noexcept;
+    [[nodiscard]] bool IsThreaded() const noexcept { return Threaded; }
 
-    virtual void VCount144() override;
-    virtual void RenderFrame() override;
-    virtual void RestartFrame() override;
-    virtual u32* GetLine(int line) override;
+    void VCount144() override;
+    void RenderFrame() override;
+    void RestartFrame() override;
+    u32* GetLine(int line) override;
 
     void SetupRenderThread();
     void StopRenderThread();
@@ -65,13 +66,13 @@ private:
     class Interpolator
     {
     public:
-        Interpolator() {}
-        Interpolator(s32 x0, s32 x1, s32 w0, s32 w1)
+        constexpr Interpolator() {}
+        constexpr Interpolator(s32 x0, s32 x1, s32 w0, s32 w1)
         {
             Setup(x0, x1, w0, w1);
         }
 
-        void Setup(s32 x0, s32 x1, s32 w0, s32 w1)
+        constexpr void Setup(s32 x0, s32 x1, s32 w0, s32 w1)
         {
             this->x0 = x0;
             this->x1 = x1;
@@ -123,7 +124,7 @@ private:
             }
         }
 
-        void SetX(s32 x)
+        constexpr void SetX(s32 x)
         {
             x -= x0;
             this->x = x;
@@ -139,7 +140,7 @@ private:
             }
         }
 
-        s32 Interpolate(s32 y0, s32 y1)
+        constexpr s32 Interpolate(s32 y0, s32 y1) const
         {
             if (xdiff == 0 || y0 == y1) return y0;
 
@@ -161,7 +162,7 @@ private:
             }
         }
 
-        s32 InterpolateZ(s32 z0, s32 z1, bool wbuffer)
+        constexpr s32 InterpolateZ(s32 z0, s32 z1, bool wbuffer) const
         {
             if (xdiff == 0 || z0 == z1) return z0;
 
@@ -228,9 +229,9 @@ private:
     class Slope
     {
     public:
-        Slope() {}
+        constexpr Slope() {}
 
-        s32 SetupDummy(s32 x0)
+        constexpr s32 SetupDummy(s32 x0)
         {
             dx = 0;
 
@@ -249,7 +250,7 @@ private:
             return x0;
         }
 
-        s32 Setup(s32 x0, s32 x1, s32 y0, s32 y1, s32 w0, s32 w1, s32 y)
+        constexpr s32 Setup(s32 x0, s32 x1, s32 y0, s32 y1, s32 w0, s32 w1, s32 y)
         {
             this->x0 = x0;
             this->y = y;
@@ -293,7 +294,7 @@ private:
 
             XMajor = (Increment > 0x40000);
 
-            if (side)
+            if constexpr (side)
             {
                 // right
 
@@ -324,7 +325,7 @@ private:
             return x;
         }
 
-        s32 Step()
+        constexpr s32 Step()
         {
             dx += Increment;
             y++;
@@ -334,7 +335,7 @@ private:
             return x;
         }
 
-        s32 XVal()
+        constexpr s32 XVal() const
         {
             s32 ret;
             if (Negative) ret = x0 - (dx >> 18);
@@ -346,7 +347,7 @@ private:
         }
 
         template<bool swapped>
-        void EdgeParams_XMajor(s32* length, s32* coverage)
+        constexpr void EdgeParams_XMajor(s32* length, s32* coverage) const
         {
             // only do length calc for right side when swapped as it's
             // only needed for aa calcs, as actual line spans are broken
@@ -372,7 +373,7 @@ private:
         }
 
         template<bool swapped>
-        void EdgeParams_YMajor(s32* length, s32* coverage)
+        constexpr void EdgeParams_YMajor(s32* length, s32* coverage) const
         {
             *length = 1;
 
@@ -404,7 +405,7 @@ private:
         }
 
         template<bool swapped>
-        void EdgeParams(s32* length, s32* coverage)
+        constexpr void EdgeParams(s32* length, s32* coverage) const
         {
             if (XMajor)
                 return EdgeParams_XMajor<swapped>(length, coverage);