Conditional compilation tagging and old function cleanup

[YellowAfterlife]

What's here:

1. I removed a number of GMS1-only functions, mainly focusing on ones that cross-reference each other and would not be removed by "remove unused functions"

2. I added conditional compilation comment-tags as per GameMaker PR 2826.
   Combined with algorithm improvements, this reduces the runtime in an empty-ish project to little more than math helpers, main loop, and room handling code.

3. 2d/GL functions are now generally defined as

   function fun() {}
   // ...
   // @if feature("2d")
   fun = fun_2D;
   // @endif
   // ... in GL init
   fun = fun_GL

   which makes it easier for "remove unused functions" algorithm to strip them fully (unlike var fun or var fun, fun2, fun3)

There's quite a number of commits here, but most of them are just comment-tags being added around function groups.