For non-rd mode (speed >=5): use mask based on prediction block size, and
(for non-screen content mode) allow for checking horiz and vert intra modes
for blocks sizes < 16x16.

Avg psnr/ssim metrics go up by about ~0.2%.

Only allowing H/V intra on block sizes below 16x16 for now, to keep
encoding time increase very small, and also when allowing H/V on 16x16 blocks,
metrics went down on a few clips which need to be further examined.

Change-Id: I8ae0bc8cb2a964f9709612c76c5661acaab1381e