Fix issues with clearing deleted attachments Deleting an object that was acting as a framebuffer color attachment in the current framebuffer didn't previously update all of the framebuffer state correctly. Fix this by going through the usual resetAttachment path when any framebuffer attachment is deleted instead of having custom code that only updated part of the state. Also early out from clearbuffer calls in case of a missing color buffer - even now that the draw buffer mask is being updated correctly, some backend code doesn't take it into account. One example is querying attachment format when the SRGB clear for linear framebuffer attachments workaround is active. BUG=angleproject:2831 TEST=angle_end2end_tests Change-Id: I1071a60dc0251946fed00e88e43a244fe59f4863 Reviewed-on: https://chromium-review.googlesource.com/1235656 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>