Fix calculation of OBMC lap dimensions

Individual OBMC lapped predictions have a max width of 64 pixels
for the top lap and have a max height of 64 for the left laps

This is 7.11.3.9. Overlapped motion compensation process
step4 = Clip3( 2, 16, Num_4x4_Blocks_Wide[ candSz ] )

dav1d wasn't clipping this as needed, which means that with scaled MC, the
interpolation of the 2nd half of a 128 block was incorrect, since mx/my
for subpel filter selection need to be reset at the 64 pixel boundary