Corrected usage of image strides

The function vp8_post_proc_down_and_across_c takes the
stride of both the src and dst images as parameters, but
assumes that they are the same.

I modified the code to use the correct strides, as the
assembler versions of these functions do.

Change-Id: I222715b774cd071b21c15a4b0d2f4aef64a520de
1 file changed