Support trapezoidal models for global motion

Adds functinoality for least-squares, RANSAC as well as encoding and
decoding with new constrained homographies that warp blocks to horizontal
and/or vertical trapezoids. This is for future experimentation. None
of the models are actually enabled in the code.

Change-Id: I1936018c6b11587d6fd83c3a2c63548cb641b33f
12 files changed