Note that glReadPixels() does not perform sRGB conversion.

Change-Id: I3f9089b79652ce42cb5695d5b6a8ce92d15c27a8
Reviewed-on: https://swiftshader-review.googlesource.com/14492Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>