The SSE3 convolve functions do not work when scaling is involved. If
either x_step_q4 or y_step_q4 is not 16 scaling will occur and an assert
fails.

This patch creates a C version of av1_convolve, av1_convolve_c. This
function in turn calls all C versions of what av1_convolve calls. New C
versions of these called functions have been created where needed and
the same for the functions they call. This means that when scaling is
enabled no asserts fail. av1_convolve_c is called instead of
av1_convolve when x_step_q4 or y_step_q4 is not 16.

Change-Id: I604d2716e034e23a0553fb7004133d3075514a7a