The following enables the renderer build on darwin.
Moved platform specific type declarations from render_api.h
to render_api_platform_types.h so it can be included from the
objective c code.
Fixed subwindow to use EmuGLView which overrides NSView to prevent
background drawing of the view.
The application provides the window handle to which the
OpenglRenderer should render to however only a sub-region of
this window needs to be rendered. This change adds this functionality
by creating a native child subwindow into which rendering will happen.