Implements a first version of global motion where the
existing ZEROMV mode is converted to a translation only
global motion mode.
A lot of the code for supporting a rotation-zoom affine
model is also incorporated.
WIP.
Change-Id: Ia1288a8dfe82f89484d4e291780288388e56d91b
Values of MODE_UPDATE_PROB and VP9_COEF_UPDATE_PROB are equal, so replacing
them with one constant. Inlining appropriate arguments for functions:
vp9_cond_prob_diff_update (encoder)
vp9_diff_update_prob (decoder)
Change-Id: I1255a1cb477743b799b3bfbbcd8de6b32b067338