introduce VP8EncQuantize2Blocks to quantize two blocks at a time

No speed diff for now. We might reorder better the instructions later,
to speed things up.

Change-Id: I1949525a0b329c7fd861b8dbea7db4b23d37709c
add a DispatchAlpha() for SSE2 that handles 8 pixels at a time

Only slightly faster.

Change-Id: Ie2e57e6a0950166124cf1075c6c9b45b7abdad8c
Merge "add a DispatchAlpha() for SSE2 that handles 8 pixels at a time"