Fix & cleanup advanced blend equations support * Made advanced blend equation extensions enableable to facilitate better testing. * Made initial BLEND_ADVANCED_COHERENT state to be unconditionally true regardless of the extension support. If the coherent extension is unsupported, that state must not be observable anyway. * Ensured that advanced blend equation functionality is supported on unextended OpenGL ES 3.2 contexts. * Removed redundant ES 2.0+ context version checks because these extensions must never be enabled on ES 1.x contexts. * Fixed ValidateBlendBarrierKHR to set the correct error code and properly mark invalid calls. * Added more negative test cases and enabled tests on OpenGL ES 3.0 contexts. Bug: angleproject:42262258 Bug: angleproject:406922380 Change-Id: I709376ad40b09972a31a691ca3813e87dd2c8165 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6421857 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>