LUMA texture workarounds are now being used correctly in case texture
levels that have only been touched by generateMipmap are used to draw.

BUG=angleproject:2498
TEST=angle_end2end_tests

Change-Id: I739550d59f99a4a22ffae766a5ee52c07610d78a
Reviewed-on: https://chromium-review.googlesource.com/1025995Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>