UI: Skip unnecessary cursor setting

Currently, in sculpting, weight paint and vertex paint modes every cursor
movement triggers redraw of a brush. During that redraw, native cursor is set.
Under the hood, setting the cursor causes freeing of previous cursor and
allocating a new one. In most cases, in previously mentioned modes, recreating
cursor is unnecessary since cursor stays the same.

This patch adds a check which skips cursor change if requested cursor is
already set. The check could be added in pain_cursor.c, but I felt adding it
inside WM_cursor_set function would hopefully skip more unnecessary cursor
reallocations.

Differential Revision: https://developer.blender.org/D7828

Reviewed by: Julian Eisel