Fix Texture2D PVRTC size computation

PVRTC imagesize computation was inverted between PVRTC 2bpp and PVRTC 4bpp (see Issue 6 in http://www.khronos.org/registry/gles/extensions/IMG/IMG_texture_compression_pvrtc.txt for source)

Also added missing support to initialize a Texture2D with ETC1 surfaceformat