Fix two issues with GLES2 devices * make sure maxColorAttachments is greater than 0 * Do not sync read buffer if maxDrawBuffers is 0 Bug: chromium:1167731 Change-Id: Icd0fb5e65ec6ffdc0d6f6325ccd1207958922254 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2733268 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Peng Huang <penghuang@chromium.org>