TabletTool: use "pencil" cursor shape

This makes tablet tool cursors visually distinct from pointer cursors by
default. Client may of course continue to set custom cursors for tablet
tools if they have focus.

Also fixes a custom cursor set by a client persisting after the tablet
exits the client's surface until proximity out.

river/TabletTool.zig

@@ -186,7 +186,7 @@ pub fn proximity(tool: *TabletTool, tablet: *Tablet, event: *wlr.Tablet.event.Pr
 
             tool.wlr_cursor.warpAbsolute(tablet.device.wlr_device, event.x, event.y);
 
-            tool.wlr_cursor.setXcursor(tablet.device.seat.cursor.xcursor_manager, "default");
+            tool.wlr_cursor.setXcursor(tablet.device.seat.cursor.xcursor_manager, "pencil");
 
             tool.passthrough(tablet);
         },
@@ -259,6 +259,8 @@ fn passthrough(tool: *TabletTool, tablet: *Tablet) void {
             tool.wp_tool.notifyMotion(result.sx, result.sy);
             return;
         }
+    } else {
+        tool.wlr_cursor.setXcursor(tablet.device.seat.cursor.xcursor_manager, "pencil");
     }
 
     tool.wp_tool.notifyProximityOut();