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/NDS.h

@@ -19,6 +19,7 @@
 #ifndef NDS_H
 #define NDS_H
 
+#include <memory>
 #include <string>
 #include <memory>
 #include <functional>
@@ -40,6 +41,11 @@ class Wifi;
 
 class AREngine;
 
+namespace Melon
+{
+class GPU;
+}
+
 namespace NDS
 {
 
@@ -262,6 +268,7 @@ extern class RTC* RTC;
 extern class Wifi* Wifi;
 extern std::unique_ptr<NDSCart::NDSCartSlot> NDSCartSlot;
 extern std::unique_ptr<GBACart::GBACartSlot> GBACartSlot;
+extern std::unique_ptr<Melon::GPU> GPU;
 extern class AREngine* AREngine;
 
 const u32 ARM7WRAMSize = 0x10000;