Add support for transcoding to/from any supported format

ManagedPVRTC (used in MonoGame) only support compressing to PVRTC. PVRTexLib actually support transcoding/compressing to and from a large number of formats including DXT and ETC1 (the later is guarantied to be supported by all Android devices).

PVRTexLib can be used for all texture processing needs in order to ensure consistent result across all platforms.

This extra API is a first step to improve texture compression (and general processing) support for MonoGame. It is implemented as a separate `Transcode` function (might be better to call it `Process` since it could be used to perform resize or mipmap only operations as well). 

Hopefully this PR can start a discussion :) I am willing to spend some time on this so all feedback welcome.