Add support for iOS application types in the MipMapGenerator

Added support for iOS application types in the MipMapGenerator utility class.  This tells libGDX to use the OGL ES2 glGenerateMipmap() call to create mipmaps, instead of the CPU-based code.  One major advantage to this fix is that you can now use non-square textures on iOS.

TESTED: built and ran on iOS simulator and on an iPknoe 4 using a small test app that applies a single-channel non-square (but POT, it was 1024 x 512) texture to the faces of a cube.