All GL RGB formats that ANGLE stores as RGBA formats under the covers
need to have their alpha channel set to 1. See GLES 3.0.4 table 3.24
for the relevant spec. In some cases, this is handled by the
dataInitializerFunction associated with the format. Previously, some
texture formats had the function set correctly, but not all.

Associating formats with the datainitializerFormat is now a
responsibility of the gen_texture_format_table.py script. The new
automation there makes sure that all GL RGB formats emulated with
RGBA formats get a data initializer function.

Tests are added in end2end_tests for most of the RGB formats.

BUG=angleproject:1318
TEST=angle_end2end_tests

Change-Id: Iad860357f33d87c625445ea6c58a53af47e0b547
Reviewed-on: https://chromium-review.googlesource.com/328253Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>