Optimize bilinear sub-pixel filters in sse2

Using bilinear filters could speed up the codec in real-time mode.
This patch added sse2 optimizations of bilinear filters that
operate on different-sized blocks.

Tests showed that the real-time encoder was speeded up by 3%.

Change-Id: If99a7ee4385fcc225c3ee7445d962d5752e57c3f