Users gets confused when running shader examples

[Larpon]

Related: #16113

Users sometimes trip when trying to run V code that uses shaders. The main reason seems to be the error message about the missing header #include that contains all the platform specific boilerplate code for bringing the shader into V/C code.

It would be nice to either be able to give a better error message or let the compiler handle everything needed seamlessly.

[JalonSolov]

It might be more than we want, but... some code could be added to vinit() when the compiler sees the "#include glsl.h" line...

[Larpon]

But we'd need heuristics to determine if the header is shader code (not all files have a sane name), and heuristics tend to be fragile and error prone. The include directive could maybe be expanded with specific support for .glsl file endings?