Warp pointer on first/last pixel

Previously, the pointer would get warped when it had left the window.
This doesn't work when sxiv is run in fullscreen mode (or similar -- see
dwm's monocle mode): The pointer can never leave the window in such
situations.

To fix this, warp the pointer when it's on the first/last pixel. To
avoid an endless warping loop, the new position must be at least one
pixel away from the next warp position.

commands.c

@@ -352,14 +352,14 @@ bool i_drag(arg_t a)
 				y = e.xmotion.y;
 
 				/* wrap the mouse around */
-				if (x < 0) {
-					WARP(win.w, y);
-				} else if (x > win.w) {
-					WARP(0, y);
-				} else if (y < 0) {
-					WARP(x, win.h);
-				} else if (y > win.h) {
-					WARP(x, 0);
+				if (x <= 0) {
+					WARP(win.w - 2, y);
+				} else if (x >= win.w - 1) {
+					WARP(1, y);
+				} else if (y <= 0) {
+					WARP(x, win.h - 2);
+				} else if (y >= win.h - 1) {
+					WARP(x, 1);
 				}
 				dx += x - ox;
 				dy += y - oy;