The viewport flipping we use would give incorrect values. Correct
them using the same logic we do for gl_PointCoord.

Also corrects the viewport scale driver uniform. It was not being
updated after the draw framebuffer setting was adjusted.

Also corrects the naming of the replaced builtin variables so it
can't possibly conflict with a user variable.

This was affecting the OpenGL line raster emulation.

Bug: angleproject:2598
Change-Id: I843b5ac7b02160a5ec81a6f8ed2d937b0937198b
Reviewed-on: https://chromium-review.googlesource.com/1208515Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>