Additional warnings found with more testing and added C4267 warning disable only for angle_libpng

BUG=angleproject:1120

Change-Id: Ic403dcff5a8018056fa51a8c408e64207f3362eb
Reviewed-on: https://chromium-review.googlesource.com/293028Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>