Flag to control emission of line directives

[shader-slang]

Flag to control emission of line directives

[tangent-vector-personal]

One more detail here is that we might want to either:

• switch to always using proper C-style line directives, even for GLSL output, or
• give the user a way to control the directive style

This is because glslang at least supports better line directives, so we might as well take advantage of it to produce better error messages in rewriter mode

[tangent-vector]

Pull request #132 added a reasonable level of control for #line directives through the API interface, and I'd like to call that good enough for now. If users want to resurrect this issue, they can.