GLSL layout rules for uniform and buffer blocks

[tangent-vector]

There is code in TypeLayout.cpp that tries to implement the std140 and std430 rules, but I have little confidence that it is being invoked correctly.

Tasks:

• Set up some reflection-generation tests for GLSL, so we can see how layouts are being computed

  • Need to be careful when defining the expected output here; should probably run the same input through glslang when generating baselines to double-check offsets

• Ensure that we are picking up the rules specified as a layout attribute and applying them correctly.

• Ensure that given GLSL source we are picking appropriate rules by default when nothing is specified (e.g., std140 for all uniform blocks, and std430 for all buffer blocks)

• Make sure to emit downstream code that reflects the layout choices we make, either by applying a layout attribute to the block, or by applying layout(offset=...) to each member. We should be conservative and try not to require too many extended features that could make it harder to output portable OpenGL GLSL later.

[tangent-vector-personal]

Pull requests #56 and #58 covered this. Closing for now. New issues can be filed if anything else comes up.