Removes low levels for I420ToBGRA and I420ToRAW and reimplements them as I420ToRGBA and I420ToRGB24 with transposed color matrix.

Adds unittests that do 1 step conversion vs 2 steps to test end swapping versions match direct conversions.

R=harryjin@google.com
BUG=libyuv:518

Review URL: https://codereview.chromium.org/1427993004 .