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.cpp

@@ -18,6 +18,9 @@
 
 #include "OpenGLSupport.h"
 
+namespace melonDS
+{
+
 using Platform::Log;
 using Platform::LogLevel;
 
@@ -142,3 +145,5 @@ void UseShaderProgram(GLuint* ids)
 }
 
 }
+
+}