Add an optional EGL_D3D11_TEXTURE_FORMAT_ANGLE attribute that is the
GL internal format to use for EGLImage being created.  This will be used
for RGB emulation of swap chain texture backed WebGL contexts in Chrome
when they have alpha:false set.  Without RGB emulation it is possible to
observe side-effects of the underlying swap chain being RGBA such as
reading/writing the alpha channel, BlitFramebuffer working/not working
when expected, etc.

Also document creating EGLImages from D3D11 textures in the existing
EGL_ANGLE_d3d_texture_client_buffer extension along with RGB emulation.

Bug: chromium:699566, chromium:939657
Change-Id: I4931cb7bdc46e9bc6debd56b79ecc10ea27bd78b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1777099Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>