convolve: support larger blocks, fix asm saturation bug

Updates the common convoloution code to support blocks larger than
16x16, and rectangular blocks. This uncovered a bug in the SSSE3
filtering routines due to the order of application of saturation.
This commit fixes that bug, adjusts the unit test to bias its
random values towards the extremes, and adds a test to ensure that
all filters conform to the expected pairwise addition structure.

Change-Id: I81f69668b1de0de5a8ed43f0643845641525c8f0