WindowSurfaceCGL does quite a bit of emulation using GL textures and
renderbuffers but some of these operations need to happen during EGL
functions when there may be no native context current.

Add a helper that ensures a context is current for operations that
manipulate GL objects.

Bug: angleproject:5138
Change-Id: Ic9f87aa26fd178a40510836c3aca8814382f92e5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2456051Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>