Support PKM Files with ETC1 Texture Compression on Android

As the title says, this adds support for ETC1 texture compression on Android. It is tested with PKM files generated using `etc1tool` from the Android SDK. There is no way to check if the device actually supports ETC1 (but most of them do), so it is a good idea to also set [&lt;supports-gl-texture&gt;](http://developer.android.com/guide/topics/manifest/supports-gl-texture-element.html) in the manifest of your host when relying on ETC1.

Besides adding support for ETC1 textures, this also refactors the way PVR textures (supported on iOS) are handled. So, in total, this pull request is quite large, but I hope the individual changes to the files are still comprehensible.