Shader uniforms

[matteblair]

Adds functionality for setting shader uniforms by name and fixes some previous flaws in ShaderProgram; Further possible work:

• Add convenience methods for setting uniforms from GLM vector and matrix types (possible now, but slightly verbose)
• Store uniform values in ShaderProgram and make glUniform calls only when value is changed (an optimization that seems premature at this point)
• Uniform arrays? (They seem to be very rarely used)

[bcamper]

This is where the dynamic JS methods are nice, with just one uniform method with variable types:
https://github.com/tangram-map/tangram/blob/master/src/gl/gl_program.js#L282

That then get dispatched to the appropriate GL uniform setter variant:
https://github.com/tangram-map/tangram/blob/master/src/gl/gl_program.js#L309

[matteblair]

Yeah that made me jealous haha. No such luck for poor old statically compiled languages (or at least nothing faster than typing these up)