- Rename ED_view3d_win_to_delta `mval` argument to `xy_delta` as it
as it was misleading since this is an screen-space offset not a region
relative cursor position (typical use of the name `mval`).
Also rename the variable passed to this function which also used the
term `mval` in many places.
- Re-order the output argument of ED_view3d_win_to_delta last.
use an r_ prefix for return arguments.
- Document how the `zfac` argument is intended to be used.
- Split ED_view3d_calc_zfac into two functions as the `r_flip` argument
was only used in some special cases.
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