Metal: End the current render pass at glInvalidateFramebuffer. Prior to this, glInvalidateFramebuffer calls were discarding the effects of draw calls made afterward, in the same render pass. Add StateChangeTestWebGL2.InvalidateThenDrawFBO reproducing the basic problem reported, and document the variations attempted. Minimal code was changed to fix this issue because other changes hypothesized as necessary weren't testable. Drive-by cleanup of FragDepthTest, after which StateChangeTestWebGL2 was patterned. Bug: angleproject:6923 Change-Id: If79805dbf62fae76b7b9147940b5d9d301142baa Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3426828 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Gregg Tavares <gman@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>