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/OpenGLSupport.h

@@ -25,8 +25,7 @@
 #include "Platform.h"
 #include "PlatformOGL.h"
 
-
-namespace OpenGL
+namespace melonDS::OpenGL
 {
 
 bool BuildShaderProgram(const char* vs, const char* fs, GLuint* ids, const char* name);