Blender Git Loki

Fix T46340: Auto scrolling in text editor with upward selection not work.

Cleanup and rework a bit text_cursor_set_to_pos_wrapped(), it did not handle correctly negative y valuesi,
because it was only checking lines starting from first visible one.

Mostly, we now directly get line matching given y position (be it an invisible one) from the helper
(renamed `get_line_pos_wrapped()`), instead of first visible one, which allows us to get rid of
additional complexity of looping over next lines until we find correct one.

This code remains rather complex to follow, added some asserts to ensure everything works as expected,
and tested it rather seriously, but DO NOT backport this to 2.76!