Use C99 types internally but define old types for compatibility
with older API.  (r1690 and earlier)

TBR=braveyao@chromium.org
Bug: libyuv:774
Test: try bots build on all platforms
Change-Id: I06f89537da3875f74e65189897e67b69af2c2ec2
Reviewed-on: https://chromium-review.googlesource.com/882501Reviewed-by: Frank Barchard <fbarchard@chromium.org>