Emulate the LUMINANCE, ALPHA and LUMINANCE_ALPHA formats with
GL_RED/GL_RG textures on platforms that don't support the
OES_texture_{half_}float extensions. Fix a preexisting bug in
TextureGL's luminance/alpha emulation where it would use desktop
GL functionality on GLES.
This change makes WebKit on iOS pass WebGL's
conformance/extensions/oes-texture-{half-}float tests per
https://bugs.webkit.org/show_bug.cgi?id=210524 .
Bug: angleproject:4636
Change-Id: I6c158058e4b170b16ece578a3930c230de16a9ca
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2203283Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
