core+platforms, revise pico pen handling

the MSB in the tablet position register denotes pen on surface.
add logic and a hotkey for handling pen not on surface. This allows
for repositioning of the pen without the storyware reacting to it.

pico/pico/pico.c

@@ -36,6 +36,8 @@ static void PicoResetPico(void)
   PicoPicohw.fifo_bytes = 0;
   PicoPicohw.r12 = 0;
 
+  PicoPicohw.pen_pos[0] = PicoPicohw.pen_pos[1] = 0x8000;
+
   PicoPicoPCMIrqEn(0);
   PicoPicoPCMFilter(0);
   PicoPicoPCMGain(8);