[Cogl] Updated patchset for depth-texture support

[Robert Bragg]

Ok, I'm quite keen on getting this depth-texture support landed in master
soon, so I've taken another pass over these patches...

In particular I was interested in better understanding the DEPTH_STENCIL
problem to see if we could maybe do away with the patch titled "Use the
GL_DEPTH24_STENCIL8 internal format for packed buffers" which didn't
sound necessary based on Neil's comments.

I have an updated patchset rebased on master now that I hope is ready to
land so I'd be grateful for any review please.

Of the patches that follow, please note that 4 of them are titled "tweak:
Support retrieving depth textures from framebuffers" and the intention
for those is to fold them back into Damien's "Support retrieving depth
textures from framebuffers" patch and I kept them separate just for
review purposes.

kind regards,
- Robert