checker,gen: allow using methods as function pointers

[spaceface777]

No description provided.

[spytheman]

Brilliant!

[spytheman]

It could even run this:

module main

import gg
import gx

[heap]
struct App {
mut:
	gg     &gg.Context = 0
	radius f64 = 10.0
}

fn (mut app App) frame(x voidptr) {
        // NB: here x is === voidptr(app), and gg will call the method 🎉 ...
	app.gg.begin()
	app.gg.draw_circle_empty(150, 150, f32(app.radius), gx.blue)
	app.gg.draw_text(20, 20, 'radius: $app.radius')
	app.gg.end()
}

fn main() {
	mut app := &App{}
	app.gg = gg.new_context(
		bg_color: gx.white
		width: 300
		height: 300
		window_title: 'Circles'
		frame_fn: app.frame // passing a method as a callback works fine!
		user_data: app
	)
	app.gg.run()
}

[larpon]

Fantastic! 🥳