Cut performance cost in QSGContext::prepareMaterial()

Cut performance cost in QSGContext::prepareMaterial()

The hash-lookup in this function is costing us a lot, and
since we're looking up the same materials again and again,
and the material has a place holder for it, we can store it
directly in the material at no extra cost. This was a 10%
gain in this particular benchmark.

Change-Id: I46b67791ce39f453fa86d1ee82f6f5c7785b46a1
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>