[3.x] Fix TextEdit color_region_cache bug

[lawnjelly]

Due to a single error, _is_line_in_region was previously iterating over the color highlighting for the entire document repeatedly for each line in the script.

This is now fixed, which should make the editor much faster with large scripts.

Explanation

The loop is meant to find the first line that is in the cache, but due to the error, it repeatedly checks whether the same line is in the cache, so it never breaks, and always reaches the beginning, so previous_line is incorrect throughout the rest of the routine.

Partially fixes #74733

Notes

This bug was really bad for performance - debug logging revealed that for each line of script the syntax highlighter evaluated every previous line, each time, so the performance drop was with the square of the number of lines, something like that.
i.e.

_is_line_in_region(5)
... evaluates 0, 1, 2, 3, 4
_is_line_in_region(6)
... evaluates 0, 1, 2, 3, 4 AGAIN and then 5
etc etc

• I'll have a look if the same bug exists in master - no, looks like the CodeHighlighter there is correct in the same place.
• Bug was introduced in Fixed colour regions and added local colour region cache #18040 so has been present for 5 years. 😀

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.5.3.