Revive geometry shaders

Fixes the bit rot that has set in for geometry shaders. They're to a
point now that we can use them again for GL experiments. Since SkSL
does not support geometry shaders yet, we pass the shader string
directly to the GL compiler for now.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5080

Change-Id: Ie3864e5559c810c682cf5f1709bdab87f033c43b
Reviewed-on: https://skia-review.googlesource.com/5080
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
10 files changed