Instead of using a single varying packing for all program stages,
we switch to using a varying register packing for each pair of
input/output shaders. This allows several valid use cases that use
many varying to succeed. For instance Geometry Shaders have both
an input and output varying packing. With tessellation shaders the
upper bound of valid varying packings in one Program goes up even
more.

We keep multiple varying packings at once inside a new
"ProgramVaryingPacking" class. Internally the class keeps a unique
varying mapping for each input/output interface in the program.
Separable programs with "open" interfaces are handled specially.

Fixes a bug where varying counting was artificially limited for
programs with more than two shaders.

This CL also disables GS support when we're emulating line raster
so we don't have to figure out the details on how to place the
special position varying.

Bug: angleproject:5496
Change-Id: I1f9a327c4750caef570c608d86953e9d0cc5eea3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2606532
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>