Refactor: BLF Offset from Cursor Position Early Exits

BLF function blf_str_offset_from_cursor_position returns the character
offset in a string given a horizontal cursor position. It currently has
no early exits, but can do so for no string or empty string. For negative
position values (so left of the string), do not exit out with zero but
instead act as if it were zero to ensure we test against the first glyph,
which might not be part of the character.

@@ -1030,1 +1034,4 @@
 
+  /* For negative position, don't early exit with 0 but instead test as
+   * if it were zero. First glyph might not be from first character. */
+  location_x = std::max(location_x, 0);