shadow: Only use correctly sized textures for shortcut

When creating a shadow for a ClutterTexture, we currently use the
underlying CoglTexture directly instead of rendering the actor to
an offscreen buffer. This assumes that the CoglTexture is directly
suitable as shadow source, which isn't necessarily the case - it
may have a very different size than what is shown and scaled up or
down by the hardware. In that case we end up with a scaled shadow
texture as well, which messes up the desired blur effect - the
result will be too light when scaling up, or too sharp when scaling
down. To fix this, only take the shortcut when a ClutterTexture's
underlying texture has the correct size and fall back to offscreen
rendering otherwise.

https://bugzilla.gnome.org/show_bug.cgi?id=788039