WIP: ssse3 version of convolve avg functions

Initial ssse3 convolve avg functions and is one step closer
to using x86inc.asm.  The decoder performance improved by 8% for
the test clip used.  This should be revisited later to see if
averaging outside the loop is better than having many similar
filter functions.

Change-Id: Ice3fafb423b02710b0448ffca18b296bcac649e9