Don't use reserved keyword "texture" as uniform variable name

The "texture" keyword is a function name in OpenGL core profile. This
commit is in preparation for making the Qt Quick 2 renderer and
materials work with a core profile context.

Change-Id: Iad243e64ab8db739fc46b85bb626bdb8b9ceb208
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>