GLSL Varying Attributes

The OpenGL GLSL target will run a Halide Func on a screen filling quad inside the fragment shader of a GLSL program. This branch adds passes during lowering and GLSL codegen to identify linear expressions that may be pulled out of the fragment shader. These expressions are evaluated per-vertex, as varying attributes, and then interpolated across the image by the GPU. The branch also contains changes to pack scalar parameters into vec4 uniform and varying attributes.

The code in this branch will interact with the specialized_branched_loops branch, to support piecewise linear expressions.  It will also interact with the upcoming GPU runtime API refactoring.

PTAL
I've added end-to-end correctness tests for varying attribute use cases in the test opengl_varying. The functionality is also tested by opengl_lut.

opengl_lut fails due to #558 in both the branch and master
opengl_varying fails due to #559 in both the branch and master