Skip indexed clears on disabled draw buffers Fixes ClearTestES3.ClearDisabledNonZeroAttachmentNoAssert Also fixed an assert in RenderTargetCache::updateColorRenderTarget Bug: angleproject:4607 Change-Id: Ic527eabacd424786736876136590b8409c9b49d1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2172091 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>