Add palette coding mode for UV

Add palette coding mode for UV

For 444 videos, a single palette of 3-d colors is
generated for YUV. For 420 videos, there may be two
palettes, one for Y, and the other for UV.

Also fixed a bug when palette and tx-skip are both on.

on derflr
--enable-palette                    +0.00%
with all experiments                +5.87% (was +5.93%)

on screen_content
--enable-palette                    +6.00%
--enable-palette --enable-tx_skip   +15.3%

on screen_content 444 version
--enable-palette                    +6.76%
--enable-palette --enable-tx_skip   +19.5%

Change-Id: I7287090aecc90eebcd4335d132a8c2c3895dfdd4