Fix: update() called before setup() on startup

[triss]

Summary

Fixes #26

• love.update(dt) ran on startup and triggered user's update() before setup() had been run. This meant state initialised in setup() may not have been ready for update.

Lifecycle of applications should be:

setup()
update()
draw()
...

not

update()
setup()
draw()
update()

Solution

• Split love.update(dt)'s per-frame framework bookkeeping (mouse position, deltaTime, key, video looping) into a new L5_internal.updateFrameState(dt), which still runs unconditionally every frame - including the frame setup() runs on - so that state stays live during setup() exactly as before.
• The user's update() callback now lives in the same branch as draw(), which is only reached once setup() has completed - so update() naturally can't fire before setup(), with no flag needing to be shared across functions.

Test plan

• Reproduced the reported error (attempt to perform arithmetic on global 'v' (a nil value)) on pre-fix code using the issue's exact repro script run via love
• Confirmed the same script runs cleanly post-fix, with setup() completing before update() fires
• Confirmed mouseX, mouseY, deltaTime, and key are still live (non-default/non-nil) inside setup()
• Ran the repo's main.lua example sketch to confirm no regression in normal draw-loop behavior
• Confirm change doesn't break other examples