Turn all framebuffer objects (including driver) into GObjects

This means:

• CoglOnscreen is now a derivable
• CoglOnscreenEgl is also a derivable
• CoglOnscreenGlx inherits CoglOnscreen
• CoglOnscreenXlib inherits CoglOnscreenEgl
• MetaOnscreenNative inherits CoglOnscreenEgl (was also split away from meta-renderer-native.c)

The "driver" of a framebuffer was GObjectified too:

• CoglFramebufferDriver replaces a few vfuncs on the cogl driver vtable and is the generic layer
• CoglGlFramebuffer is the OpenGL/GLES driver for all CoglFramebuffer types and inherits CoglFramebufferDriver
• CoglGlFramebufferFbo drives FBO backend framebuffers (i.e. CoglOffscreen atm)
• CoglGlFramebufferBack drives back buffer based framebuffers (i.e. all current CoglOnscreen ones)

Currently marked as WIP as it sits on top of the atomic KMS merge request.