Avoid allocating and (failing to) manage our own texture cache

There is no reason we should be allocating and managing our own set of
window decorations. This is only going to add to the total texture
memory used and because it is managed as a shared GL resource, it
would have needed some care during cleanup to clean it up correctly.

Change-Id: I19651837da6b3dfde0f78a964982f3f67e577493
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>