The emulation of the "alpha: false" setting was done with an
RGBA format under the hood, which caused some unexpected
results, which included non constant alpha values. Replacing
GL_BGRA8_EXT with GL_BGRX8_ANGLEX solves this issue.

This was discovered while debugging the following test:
conformance/context/context-no-alpha-fbo-with-alpha.html
but it may fix many other tests (I will update the
expectations file after this fix rolls into Chromium)

Bug: chromium:1099960
Change-Id: Ib7b2d352a8e920983d3398de28a8c1dde3b97130
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3010579Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>