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/GPU3D_OpenGL_shaders.h

@@ -21,7 +21,8 @@
 
 #define kShaderHeader "#version 140"
 
-
+namespace melonDS
+{
 const char* kClearVS = kShaderHeader R"(
 
 in vec2 vPosition;
@@ -802,5 +803,5 @@ void main()
     gl_FragDepth = fZ;
 }
 )";
-
+}
 #endif // GPU3D_OPENGL_SHADERS_H