Enable PVR support on iOS

MOAI_TEST_PVR was defined in a place where the actual PVR
code could not see it. Since it was just an alias for
MOAI_OS_IPHONE, I replaced it with that.
Fix PVR support by using zgl abstraction layer
Remove unnessecary casts
Merge pull request #755 from theGoodEvil/pvr_support

Fix PVR Texture Support