vxfw: add separate function pointer for event capturing

Move capture handling of events to a different method on Widget. Having
capturing phase in the same method makes it very easy to accidentally
capture an event, producing confusing results. Browsers and GTK both
require handlers to explicitly listen to capturing phase events, so
there is precedence for having this as a separate method. For
applications that want to handle it all within the same function, the
signature is the same so they can use the same function for both methods
and achieve the same result.

src/vxfw/App.zig

@@ -261,7 +261,7 @@ const MouseHandler = struct {
             var m_local = mouse;
             m_local.col = item.local.col;
             m_local.row = item.local.row;
-            try item.widget.handleEvent(ctx, .{ .mouse = m_local });
+            try item.widget.captureEvent(ctx, .{ .mouse = m_local });
             try app.handleCommand(&ctx.cmds);
 
             // If the event was consumed, we check if we need to send a mouse_leave and return

src/vxfw/vxfw.zig

@@ -196,9 +196,16 @@ pub const MaxSize = struct {
 /// The Widget interface
 pub const Widget = struct {
     userdata: *anyopaque,
+    captureHandler: ?*const fn (userdata: *anyopaque, ctx: *EventContext, event: Event) anyerror!void = null,
     eventHandler: ?*const fn (userdata: *anyopaque, ctx: *EventContext, event: Event) anyerror!void = null,
     drawFn: *const fn (userdata: *anyopaque, ctx: DrawContext) Allocator.Error!Surface,
 
+    pub fn captureEvent(self: Widget, ctx: *EventContext, event: Event) anyerror!void {
+        if (self.captureHandler) |handle| {
+            return handle(self.userdata, ctx, event);
+        }
+    }
+
     pub fn handleEvent(self: Widget, ctx: *EventContext, event: Event) anyerror!void {
         if (self.eventHandler) |handle| {
             return handle(self.userdata, ctx, event);