This commit introduces some new framebuffer api to be able to
enable texture based depth buffers for a framebuffer (currently
only supported for offscreen framebuffers) and once allocated
to be able to retrieve the depth buffer as a texture for further
usage, say, to implement shadow mapping.
The API works as follow:
* Before the framebuffer is allocated, you can request that a depth
texture is created with
cogl_framebuffer_set_depth_texture_enabled()
* cogl_framebuffer_get_depth_texture() can then be used to grab a
CoglTexture once the framebuffer has been allocated.
87bc616d34
