GL: Add explicitFragmentLocations workaround Some drivers produce incorrect results when a fragment output has an implicit location and gl_SampleMask[] is written to. Fixed: angleproject:8308 Change-Id: I615952ef61b1cb611984ec7defb189d89ab3281c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4777702 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>