marked OpenGL state cache as 'experimental' for now, and deactivate it

There was a problem with masking which we couldn't yet reproduce / fix, so I
deactivated this feature for this release. While it's a nice performance plus,
it's not critical to have it deactivated.

More on the issue can be found here:
http://forum.sparrow-framework.org/topic/sxmaskedsprite-appears-before-everything-