GL: Flush after framebuffer change on Mac 9th gen Intel GPUs Speculative fix for crashes during flush on Mac. Mirrors the flush_on_framebuffer_change workaround in Chrome from http://crbug.com/783979 Bug: chromium:1181068 Change-Id: Ic84b1bfc44babf27016b306393da308d504d7fd9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2859984 Reviewed-by: Peng Huang <penghuang@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>