distance-to-edge for stencil analytic antialiasing.
This improves the rendering quality of stencil AAA significantly.
Additionally, this adds an approximation of Core Graphics' (macOS')
defringing filter.
Closes#73.
The new `vXDist` values *can* be deduced simply from the `vUV` values
and first-order derivatives thereof, and therefore they're technically
redundant. However, calculating them in this way increases the
complexity and ALU load in the fragment shader. It's simpler to just
compute the additional values in the vertex shader and use
interpolation.
technique similar to the new MCAA.
This new technique simplifies the code significantly by unifying lines,
curves, and transformed curves. Blog posts forthcoming.
Patrick Walton