texture-cache: Replace ClutterTexture by ClutterImage

texture-cache: Replace ClutterTexture by ClutterImage

ClutterTexture is a deprecated class that is simultaneously
an actor, and the content of the actor. Clutter's new model
is to separate painting (via ClutterContent) from actors.

Currently, StTextureCache relies on ClutterTexture to store
the loaded textures. This not only does not match the latest
practices of Clutter, but also generates various compile-time
warnings.

Port StTextureCache to store ClutterImages instead of storing
ClutterTextures. ClutterImage exposes the internal CoglTexture,
so no helpers are needed to match the current StTextureCache
API. Aspect ratio was dropped, but from my testing, it doesn't
change anything.