Refactor the GPU to be object-oriented (#1873)

* Refactor GPU3D to be an object

- Who has two thumbs and is the sworn enemy of global state? This guy!

* Refactor GPU itself to be an object

- Wow, it's used in a lot of places
- Also introduce a new `Melon` namespace for a few classes
- I expect other classes will be moved into `Melon` over time

* Change signature of Renderer3D::SetRenderSettings

- Make it noexcept, and its argument const

* Remove some stray whitespace

src/GPU2D_Soft.h

@@ -20,19 +20,25 @@
 
 #include "GPU2D.h"
 
+namespace Melon
+{
+class GPU;
+}
+
 namespace GPU2D
 {
 
 class SoftRenderer : public Renderer2D
 {
 public:
-    SoftRenderer();
+    SoftRenderer(Melon::GPU& gpu);
     ~SoftRenderer() override {}
 
     void DrawScanline(u32 line, Unit* unit) override;
     void DrawSprites(u32 line, Unit* unit) override;
     void VBlankEnd(Unit* unitA, Unit* unitB) override;
 private:
+    Melon::GPU& GPU;
     alignas(8) u32 BGOBJLine[256*3];
     u32* _3DLine;