Improve GLSL integer support

 - Adds shader types for uint.
 - Adds a cap for integer support.
 - Uses glVertexAttribIPointer for integer attribs.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1669853002

Review URL: https://codereview.chromium.org/1669853002
17 files changed