Update vp8enc.cpp

Added:
3) before inter coding checks scene change (in a very very simple way - checking difference of average chroma value). If scene changed - goes to intRA encoding instead of intER.

2) check SSIM of inter-transformed blocks (measured on GPU). If SSIM < some value (0.7 for example) tries to change it to INTRA mb (TM_B_PRED luma + TM_PRED chroma). If SIIM of INTRA mb higher - makes switch.

3) If amount of mbs changed from inter to intra higher than some value (1/4 of all mbs as example) flushes inter-coded frame and redo as intra frame.