These values are accessible already from the Surface state. Sometimes
they don't apply (e.g. GGP). Remove them to keep the code clearer.
Updates the offscreen surface constructor to pull width and height from
the attributes map instead of using parameters. Otherwise they weren't
used.

Refactoring change only.

Bug: angleproject:4078
Change-Id: I9e49eadc7116562f62bd8d11342d6b8835376719
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1895762Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>