Caused by rBbf8507414889: GPencil: New Fill Extend lines adjust with MMB
Above comit set the wrong cursors (WM_CURSOR_DEFAULT).
When releasing MMB, we need to return to WM_CURSOR_PAINT_BRUSH.
When exiting, we need to return to WM_CURSOR_DOT (since this is the
default cursor for the tool, see _defs_gpencil_paint >
generate_from_brushes)