OpenGL state sync issues as few GL commands as possible to update the
blend state regardless of an application input.

Enhanced ClearTestES3.MaskedIndexedClearMultipleAttachments
regression test.

Disabled OES_draw_buffers_indexed on Windows/AMD/OpenGL.

Bug: angleproject:4394
Change-Id: I244ac2975678bc559634152cf4eb997d9dbe83d0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2145874
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>