Add extension to Caps and code gen files.
Add to Validation of TexImage
Add all the autogen for new extension
Fix swizzle for stencil only
Add LoadFunction
Fix formatutils
Add validation
Test: angle_deqp_egl_tests
--deqp-case=GLES31.functional.stencil_texturing.format.stencil_index8_2d

Bug: angleproject:3231
Change-Id: Id59c7d183ea1658732887e99637d9c8faab938e1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2404327Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>