Add a corner case for framebuffer fetch Add a defect test to test that when a shader reads from gl_LastFragDepthARM or gl_LastFragStencilARM and no image is attached to the depth or stencil attachment, respectively, results should be undefined, instead of generating an error. Bug: angleproject:376572258 Change-Id: I7ea0e5021ec370374c4da532b0fa58277f3e959e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5999877 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>