doom: re-center mouse more often to limit it escaping

[qwx <qu7uux@gmail.com>]

doom: re-center mouse more often to limit it escaping

when a mouse motion reaches grabout's limits without crossing them, a following
event in which the mouse passes the remaining distance between grabout and the
window's limits will have the mouse jump out of the window.
this happens too frequently on terminals with a good drawing speed.

to reduce this, make grabout smaller, so that the distance to cross in one
event is larger. Dx(screen->r)/4 is sufficient for the terminals i have tested
this on.

diff -r 0a3cf47fce65 sys/src/games/doom/i_video.c
--- a/sys/src/games/doom/i_video.c	Mon Jun 27 00:36:54 2016 +0200
+++ b/sys/src/games/doom/i_video.c	Thu Jun 30 17:51:32 2016 +0300
@@ -44,7 +44,7 @@
 	draw(screen, screen->r, display->black, nil, ZP);
 
 	center = addpt(screen->r.min, Pt(Dx(screen->r)/2, Dy(screen->r)/2));
-	grabout = insetrect(screen->r, Dx(screen->r)/8);
+	grabout = insetrect(screen->r, Dx(screen->r)/4);
 
 	if((pid = rfork(RFPROC|RFMEM)) == 0){
 		kbdproc();
@@ -102,7 +102,7 @@
 		draw(screen, screen->r, display->black, nil, ZP);
 
 		center = addpt(screen->r.min, Pt(Dx(screen->r)/2, Dy(screen->r)/2));
-		grabout = insetrect(screen->r, Dx(screen->r)/8);
+		grabout = insetrect(screen->r, Dx(screen->r)/4);
 	}
 
 	scale = Dx(screen->r)/SCREENWIDTH;