Improve `poll_events` (it will be renamed to `update`)

[Andre-LA]

Ok, so, poll_events does much more than "poll events", it's turns out it became a "update the nene internal state" function, thus, it will renamed to nene:update (just as tinycoffee).

Thus, it will be a breaking change, however poll_events will stay for a while as a <deprecated> function.

[Andre-LA]

One of the improvements it's the refresh rate control.

This is how nene controls framerate: it doesn't :).

The only thing nene does it's using SDL_RENDERER_PRESENTVSYNC flag when creating the renderer.

However, this unsurprisingly have problems, although some of them are fixed by using nene.delta_time, it's not ideal.