d2ui: Add a custom widget

this allows us to encapsulate any custom render functionality into this
custom widget. It further helps us to remove the renderer from
game_controls.

d2core/d2ui/custom_widget.go

@@ -0,0 +1,32 @@
+package d2ui
+
+import "github.com/OpenDiablo2/OpenDiablo2/d2common/d2interface"
+
+// static check that CustomWidget implements widget
+var _ Widget = &CustomWidget{}
+
+// CustomWidget is a widget with a fully custom render function
+type CustomWidget struct {
+	*BaseWidget
+	renderFunc func(target d2interface.Surface) error
+}
+
+// NewCustomWidget creates a new widget with custom render function
+func (ui *UIManager) NewCustomWidget(renderFunc func(target d2interface.Surface) error) *CustomWidget {
+	base := NewBaseWidget(ui)
+
+	return &CustomWidget{
+		BaseWidget: base,
+		renderFunc: renderFunc,
+	}
+}
+
+// Render draws the custom widget
+func (c *CustomWidget) Render(target d2interface.Surface) error {
+	return c.renderFunc(target)
+}
+
+// Advance is a no-op
+func (c *CustomWidget) Advance(elapsed float64) error {
+	return nil
+}