gl_shader: fix shader dump line numbering

[illwieckz]

• reset line number at GLSL shader start
• use custom line number for the GLSL header to avoid line number duplicates
• use custom line number for the deform vertex header
• reset line count on #line 0

Written to debug:

• WIP: Rebase adaptive exposure #1914

[slipher]

We should be able to avoid inserting #line 0 and then doing string replacements later. I think the key issue here is that ProcessInserts is called on text that already has hard-coded stuff prepended. But that hard-coded stuff shouldn't contain inserts, so it would make more sense to call ProcessInserts with only the actual file contents. Then you wouldn't have the problem of not knowing where the first line is.