Renamed GLSL shaders to UpperCamelCase.

This matches the naming convention we use for OSL shaders.
Added a Grid node.
Removed glsl/constant.frag.

This should be in the previous commit, but OS X is case insensitive and somehow I can't squash them together.
Made grid width minimum > 0 to avoid OpenGL errors.

As far as I understand it, the GL spec specifies that a width of 0 may be specified, but will be clamped up to the next representable value. That doesn't seem to be the case with either the OS X or NVIDIA drivers though, where a width of 0 causes errors.

Also enabled line smoothing for the grid, and changed the default values to take better advantage of the new quality this provides.
Fixed SceneView bug whereby initial base state wasn't applied correctly.

This only showed itself when experimenting with additional settings in the base state - I've decided not to apply those for now, but it seems worth fixing this so it won't be a problem when we next do modify the base state.
Merge pull request #815 from johnhaddon/grid

Grid node