gg: add draw_convex_poly/2 and draw_empty_poly/2 methods (#8487)

examples/gg/polygons.v

@@ -0,0 +1,34 @@
+module main
+
+import gg
+import gx
+
+struct App {
+mut:
+	gg &gg.Context
+}
+
+fn main() {
+	mut app := &App{
+		gg: 0
+	}
+	app.gg = gg.new_context(
+		bg_color: gx.rgb(174, 198, 255)
+		width: 600
+		height: 400
+		window_title: 'Polygons'
+		frame_fn: frame
+		user_data: app
+	)
+	app.gg.run()
+}
+
+fn frame(mut app App) {
+	app.gg.begin()
+	app.gg.draw_convex_poly([f32(100.0), 100.0, 200.0, 100.0, 300.0, 200.0, 200.0, 300.0, 100.0,
+		300.0,
+	], gx.blue)
+	app.gg.draw_empty_poly([f32(50.0), 50.0, 70.0, 60.0, 90.0, 80.0, 70.0, 110.0], gx.black)
+	app.gg.draw_triangle(450, 142, 530, 280, 370, 280, gx.red)
+	app.gg.end()
+}