Chrome has started requesting RGB formats for IOSurfaces to support the alpha=false
flag for WebGL context creation.

BUG=chromium:995142

Change-Id: I0c9d994f2d8208e87a0d167ebc0787fc15add08f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35389Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Geoff Lang <geofflang@chromium.org>