Clamps the vertex position to the subpixel grid before interpolation.
This will give more correct results on systems that have less than
8 bits of subpixel accuracy.

Also uses a more accurate formulation for the emulation filter in the
fragment shader using dfdx and dfdy.

Fixes line raster CTS tests on SwiftShader. Still does not produce spec
conformant lines. Updates the public docs to indicate this.

Bug: angleproject:2830
Change-Id: Ib9a268df3e7d986bd2b1348be664389fe8fc0ef2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1826598
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>