fix(input): balance the draw stack on early return (A2)

[Vadim1987]

The input render loop opens with gfx.push('all') and has an early return in the middle (if not tl then return end) that skips the closing gfx.pop(). Whenever that branch is taken, one graphics stack level leaks per frame until LÖVE's stack depth cap is hit. The trigger is content whose string.ulen is nil — see the companion PR for how such content gets in and for keeping it out at the model boundary; this change makes the draw scope exception-safe regardless of content, same shape as the use_canvas fix (#37).

[dsent]

This is to replace #39