Only set pipeline resources as used when the resource changes

Only set pipeline resources as used when the resource changes

This avoids an issue on Vulkan where invalid descriptor set writes were
generated if a resource unused by the current shader was set to the same
value it already had and the shader wasn't changed.

I'm not really sure if the flags should be set in the common part at all,
since the OpenGL bckend barely uses them.  But other approaches are not
clearly superior either.