Changing the cursor would intermittently close Blender's window
(without crashing).
This happened because the size of a cursor must be the a multiple of the
scale, for themed cursor this is always true but with custom cursors
it's not.
Separate theme scale from custom cursor scale to avoid this bug.
In the future we can support Hi-DPI custom cursors, for now they're
scale is always set to 1.
a76c1ddecc