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
This change is orthogonal to the varying attribute feature- it is an attempt at a workaround for a similar issue in master that came up while testing.  I've added a test for this particular issue and filed #558 which explains the details.  The change in the branch has comments and internal_errors now as well.