Create declarative "useProgram" interface

[lucaswoj]

This pull request includes

• refactoring to make shader source rewriting for Figure out how to make disabled attributes performant in IE #1714 possible
• renaming "shaders" to "programs" where appropriate to stay true to the domain terminology
• merging gl_util and painter

I recommend going through this commit-by-commit to compartmentalize the renaming noise.

cc @ansis @mourner @jfirebaugh

[lucaswoj]

The high level intent here is to get rid of programs as singleton objects and move to a declarative useProgram interface, which can accept parameters in the future.

[ansis]

looks good to me!