This adds a few explicit casts where necessary and changes some double
constants to floats to avoid truncation.

Bug: chromium:1216696
Change-Id: Ie1a3ad94284185fb20a6feff75290cabb7ee09e8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2950320Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>