OPENGL: Move RGBA8888 (logical layout) to OpenGL specific formats.

The reason here is that GL_UNSIGNED_INT_8_8_8_8 is not supported by the GLES
standard. Thus we cannot use it outside OpenGL specific code paths.
OPENGL: Always support RGBA8888 (memory layout).
OPENGL/SDL: Default to RGBA8888 (memory layout).

This makes sure the default mode also works for OpenGL ES contexts.