cogl-framebuffer: Use the FBO extension for color sizes

cogl-framebuffer: Use the FBO extension for color sizes

OpenGL 3.0 deprecated querying of the GL_{RED,GREEN,BLUE}_BITS
constants, and the FBO extension provides a mechanism to query for the
color buffer sizes which *should* work even with the default
framebuffer. Unfortunately, this doesn't seem to hold for Mesa - so we
just use this for the offscreen CoglFramebuffer type, and we fall back
to glGetIntegerv() for the onscreen one.

http://bugzilla.openedhand.com/show_bug.cgi?id=2094