gg: add draw_polygon_filled test (#14871)

vlib/gg/testdata/draw_simple_polygons.vv

@@ -0,0 +1,61 @@
+import gg
+import gx
+
+struct App {
+mut:
+	gg       &gg.Context = 0
+	rotation f32 = f32(0)
+}
+
+[console]
+fn main() {
+	mut app := &App{}
+
+	app.gg = gg.new_context(
+		window_title: 'Simple Polygons'
+		width: 500
+		height: 500
+		bg_color: gx.black
+		event_fn: event
+		frame_fn: render
+		user_data: app
+	)
+
+	app.gg.run()
+}
+
+fn render(app &App) {
+	app.gg.begin()
+
+	color := gx.Color{
+		r: 175
+		g: 0
+		b: 0
+		a: 200
+	}
+	mut shape := 3
+	for i := 1; i < 5; i++ {
+		for j := 1; j < 5; j++ {
+			app.gg.draw_polygon_filled(100 * j, 100 * i, 30, shape, app.rotation, color)
+			shape++
+		}
+	}
+
+	app.gg.end()
+}
+
+fn event(e &gg.Event, mut app App) {
+	match e.typ {
+		.key_down {
+			match e.key_code {
+				.right { app.rotation++ }
+				.left { app.rotation-- }
+				.escape { app.gg.quit() }
+				else {}
+			}
+			print('rotation: ')
+			println(app.rotation)
+		}
+		else {}
+	}
+}