Add line numbers to merged shader sources and improve the debugging experience

[jellysquid3]

Right now, we don't add #line directives to the merged shader sources, which makes reading line numbers a lot harder. To make it easier to debug compilation errors, the following changes should be made:

• Each included shader file (and the prelude with #version and specialization constants) should be given a unique index.
• In the merged source code, #line <line number> <source file index> declarations should be added, and the original #include macro should be commented out (for debugging purposes).
• The shader compilation log should include the merged source code.

[douira]

Does "shader compilation log" mean that the entire shader source should be printed into the log?

[jellysquid3]

I think so, yes. I'm not sure where else we could put it, and the error diagnostics aren't useful without it. But it might also completely flood the log file...

[douira]

If you're only concerned about making the gl errors make sense, we could capture those and re-map the source ids to import names with some effort.

[jellysquid3]

We just want the errors to make sense. They don't, currently. If you think re-mapping the source ids would solve the problem, then I'm for it.