RAW is a big endian style RGB buffer with R first in memory, then G and B.
Convert NV21 and NV12 to RAW format.

Performance on SkylakeX for 720p with AVX2
I420ToRAW_Opt (388 ms)
H420ToRAW_Opt (371 ms)
NV12ToRAW_Opt (341 ms)
NV21ToRAW_Opt (339 ms)

SSSE3
I420ToRAW_Opt (507 ms)
H420ToRAW_Opt (481 ms)
NV12ToRAW_Opt (498 ms)
NV21ToRAW_Opt (493 ms)

C
I420ToRAW_Opt (2287 ms)
H420ToRAW_Opt (2246 ms)
NV12ToRAW_Opt (2191 ms)
NV21ToRAW_Opt (2204 ms)

Performance on Pixel 2 for 720p
out/Release/bin/run_libyuv_unittest -v -t 7200 --gtest_filter=*NV??ToR*Opt --libyuv_repeat=1000 --libyuv_width=1280 --libyuv_height=720
LibYUVConvertTest.NV12ToRGB24_Opt (1739 ms)
LibYUVConvertTest.NV21ToRGB24_Opt (1734 ms)
LibYUVConvertTest.NV12ToRAW_Opt (1719 ms)
LibYUVConvertTest.NV21ToRAW_Opt (1691 ms)
LibYUVConvertTest.NV12ToRGB565_Opt (2152 ms)

Bug: libyuv:778, b:117522975
Test: add new NV21ToRAW and NV12ToRAW tests
Change-Id: Ieabb68a2c6d8c26743e609c5696c81bb14fb253f
Reviewed-on: https://chromium-review.googlesource.com/c/1272615
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>