Correctly profile the size of loaded pixmaps

Only set the sizes when they're known, prefer request size to implicit
size (as the request size is what ends up in the cache), and don't set
the size twice.

Task-number: QTBUG-35337
Change-Id: Ie516a1cae2d9050f61362ee99cf8a6a9dd8ea3bb
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>