gg: add quit fn (#10223)

vlib/gg/gg.v

@@ -359,6 +359,11 @@ pub fn (gg &Context) run() {
 	sapp.run(&gg.window)
 }
 
+// quit closes the context window and exits the event loop for it
+pub fn (ctx &Context) quit() {
+	sapp.request_quit() // does not require ctx right now, but sokol multi-window might in the future
+}
+
 pub fn (mut ctx Context) set_bg_color(c gx.Color) {
 	ctx.clear_pass = gfx.create_clear_pass(f32(c.r) / 255.0, f32(c.g) / 255.0, f32(c.b) / 255.0,
 		f32(c.a) / 255.0)