GL: Add state validation functions to GL backend These functions validate that the driver's reported state matches ANGLE's internal state. Can be useful for debugging Bug: angleproject:3900 Change-Id: I35d15e991986dcab114939c551a88549f09bd263 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1797254 Commit-Queue: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>