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
A few comments on this:

First, It looks like the casts here are redundant, because the code immediately before this line will cast them to float.
Second, this is nitpicky, but I'd prefer to read this written with the standard Expr operators:

    return (cast<float>(v) / cast<float>(max_dim)) * 2.0f - 1.0f

Any reason not to do this?