Safeguard program binaries from filling up the filesystem.

Most QML applications will never have this problem as they use a small
amount of shaders and the cache will reach a max of 20-30 shaders.

This is only a safeguard against applications which recompile new shaders
all the time.

Change-Id: If6dbb6da4eff82e2a4e43a9414a45a7bf4ce762a
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>