Validate GL backend after dispatch table initialization This commit adds a check to ensure that the GL backend's functions have been properly initialized. This may happen with third party build toolchains, like vcpkg, which reimplement parts of the existing build system. Bug: angleproject:8195 Change-Id: Iaca2200a563c5049d90acad57785088c94b4e580 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4614645 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>