Parse override tag arguments exactly like VSFilter

**This is the master issue for all your argument splitting bugs.**

~~Creating this pull request just so we can have this master issue, even though ***the code isn’t presentable.*** And I mean it: just look at [that](https://github.com/astiob/libass/commit/722a64a265cb1a7451997e440721459ead394f99#diff-a8b8810d1537d203e24ef44e0ab00f04R253) faux C++ and Ruby. Do note and comment on my `tag` macro though; I think it’s pretty nice.~~

~~Although actually, isn’t it almost done? Maybe I’ll try to finish it quickly. Maybe.~~ So I did.

Known bugs/missing things (check/tick means it’s fixed):
- [x] `isspace` can crash on negative values (also in `ass_strtod` and `ass.c:process_info_line`)
- [x] the array code isn’t finished
- [x] `parse_vector_clip` hasn’t been updated
- [x] `mystrto*` modify the pointer
- [x] empty argument detection is wrong
- [x] `\t` right boundary is wrong
- [ ] https://gist.github.com/astiob/046395b5d99e4b0336fd, Cyberbeing/xy-VSFilter#11