Move all core types into namespaces (#1886)

* Reorganize namespaces

- Most types are now moved into the `melonDS` namespace
- Only good chance to do this for a while, since a big refactor is next

* Fix the build

src/GPU_OpenGL_shaders.h

@@ -19,6 +19,8 @@
 #ifndef GPU_OPENGL_SHADERS_H
 #define GPU_OPENGL_SHADERS_H
 
+namespace melonDS
+{
 const char* kCompositorVS = R"(#version 140
 
 in vec2 vPosition;
@@ -866,5 +868,6 @@ void main()
 
 
 
+}
 
 #endif // GPU_OPENGL_SHADERS_H