In preparation for compiling shaders early at link time, this change
reworks line raster emulation such that it uses specialization constants
instead of a preprocessor condition.  This means drawing both triangles
and lines with this program will still result in a one-time shader
compilation.

The compilation is still done at draw time in this change.

Bug: angleproject:3394
Change-Id: I0bf91398868d7f7147456533b728906b505192b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1992365
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>