Fix PVR Texture Support

On iOS, we used to support the PVR texture format, both compressed and uncompressed. I reenabled the code and updated it to used the zgl OpenGL abstraction layer.

I wasn't sure if this should go into master, but it is a bugfix for a broken feature, so I think it should. There is a slight chance this might break other platforms, if they do not support these parts of OpenGL:

* GL_LUMINANCE
* GL_LUMINANCE_ALPHA
* glCompressedTexImage2D()

But these would be easy to fix with an #ifdef in zl-gfx/zl_gfx_opengl.cpp.