Using the sourceRect for calculating the viewport and texture coordinates
was not enough when the source and destination rectangles were different
sizes (generating mipmaps).

BUG=709232

Change-Id: I2704ddf6e3da0939ad77d278ab495e53a2d9bc7d
Reviewed-on: https://chromium-review.googlesource.com/473266
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>