framebuffer: Support texture based depth buffers

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.

examples/cogl-info.c

@@ -109,6 +109,12 @@ struct {
     "GLES2 API integration supported",
     "Support for creating a GLES2 context for using the GLES2 API in a "
       "way that's integrated with Cogl."
+  },
+  {
+    COGL_FEATURE_ID_DEPTH_TEXTURE,
+    "Depth Textures",
+    "CoglFramebuffers can be configured to render their depth buffer into "
+    "a texture"
   }
 };