Cleanup MTS related functionality
Cleaning up MTS (multiple transform selection) code:
- Removed the obsolete arrays used to map intra prediction directions to transform modes (inherited from JEM)
- Substituted the quite cryptic EmtMode/EmtTrIndex notation with directly getting transform types for horizontal and vertical directions (using the existing TransType enum)
- Combined xT/xTrMxN_EMT into a single xT and xIT/xITrMxN_EMT into a single xIT function as there seemed to be no reason for the separation anymore (also the separation was done differently for forward and inverse functions - now those are balanced).
- The RExt__DECODER_DEBUG_TOOL_STATISTICS is activated in the forward transform, although doing it in the inverse transform would sound like a more logical place. However, kept it still in the forward transform function as I'm not familiar with this functionality.