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
What if someone accesses a texture in a way where the x coordinate depends on the color channel - won't that introduce a ramp? I think you're going to catch future errors (and document what kinds of IR node you're expecting) with a Ramp visitor of the form:

virtual void visit(const Ramp *op) {
  internal_error << "Wasn't expecting a ramp!";
}

The same argument applies to Loads and Calls. Otherwise the error message will be something about mismatched types in a constructor.