Plan 9: 9/pc: swcursor pointer trail

[Yaroslav Kolomiiets]

Sofware cursor on Plan 9 is implemented in 9/pc/screen.c:/^hwdraw/ kernel function.
emu/Plan9/win.c posts updates to screen image with 'y' draw function — the Plan 9's memload — which itself calls to hwdraw only if the window is partially off screen (in that case, no pointer trail appears).

Plan 9's vncv(1) also had this problem problem until addressed with /n/sources/patch/applied/vncv-curtrail back in 2013.

Not sure what should be fixed this time though: memload in Plan 9 or plan9loadimage in emu.

Cursor trails spoil interactive experience badly.

[Yaroslav Kolomiiets]

fixed locally in devdraw.c on Plan 9.