RFC: Add APPLE_texture_format_bgra8888 and support for internal format BGRA8_EXT
In EXT_texture_storage we have:
Accepted by the <internalformat> parameter of TexStorage* when implemented on OpenGL ES: ... (if EXT_texture_format_BGRA8888 or APPLE_texture_format_BGRA8888 is supported) BGRA8_EXT 0x93A1 ...
so make it possible to use this internal format.
Strangely enough EXT_texture_format_BGRA8888 doesn't mention GL_BGRA8_EXT but APPLE_texture_format_BGRA8888 does, so advertise this extension too.