Add support for lineloops.
Includes a compute shader for generating an index
buffer to draw lineloop.

Instancing turns out to be a special case for indirect draws if we
have vertex attributes that need to be emulated (e.g. divisor too
large or native vertex format not available).

Test: dEQP.GLES31/functional_draw_indirect_*
      angle_end2end_tests --gtest_filter=LineLoopIndirectTest.*/*
      dEQP.GLES3/functional_draw_*
Bug: angleproject:3564
Change-Id: I1fdabe2c8a690c8b6df9e252e1e839e08796bcca
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1834682
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>