Blender Git Loki

Cleanup: utf8 stepping functions

Various changes to reduce risk of out of bounds errors in utf8 seeking.

- Remove BLI_str_prev_char_utf8
This function could potentially scan past the beginning of a string.
Use BLI_str_find_prev_char_utf8 instead which takes a limiting
string start argument.

- Swap arguments for BLI_str_find_prev_char_utf8 so the stepping
argument is first and the limiting argument is last.
This matches BLI_str_find_next_char_utf8.

- Change behavior of these functions to return it the start or end
pointers instead of NULL, which complicated use of these functions
to calculate offsets.

Callers that need to check if the limits were reached can compare
the return value with the start/end pointers.

- Return 'const char *' from these functions
so they don't remove const from the input arguments.