Previously when using the cogl_rectangle_* family of functions with a
pipeline that doesn't have a texture for a particular layer then
validate_tex_coords_cb would bail out immediately leaving the texture
coords for that layer uninitialised. This patch changes it so that it
bails out after copying in the texture coordinates instead. This was
causing problems for pipelines that were trying to completely generate
the texture values in a CoglSnippet because they wouldn't get any
texture coordinates.

Reviewed-by: Robert Bragg <robert@linux.intel.com>