Revert "Cut performance cost in QSGContext::prepareMaterial()".

This reverts commit 0e9cd8b4098661bf611fa73a787c58c85e7d7338.

This optimization breaks ShaderEffects with changing fragment
and vertex shaders and needs some more thought.

Task-number: QTBUG-31837
Change-Id: I1abe249c65dd785825c249a7c22baee928bd76c8
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>