To account for framebuffer flipping, `dFdy()` expressions
must be changed to:

    dFdy() * ANGLEUniforms.viewportYScale

Bug: angleproject:3487
Test: dEQP-GLES3.functional.shaders.derivate.dfdy.*
Change-Id: I38f25ba37fb8c5ae61cee5ac911df88ec4a93fef
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1678404
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>