Improve FPS demo

[syncopika]

finally some improvements (I think) for the FPS demo which include:

• player model rotates (or looks at) towards mouse cursor on mousemove over the canvas
• better first-person view (but still not great with crosshair placement - would like to figure that out eventually)

issues:

• the animation when transitioning between holding the rifle <-> idle can look a bit wonky though :/. will hopefully look into that more eventually

also added some minor edits for abacus demo to make it more mobile-friendly (e.g. pointermove evt support via touch-action: none)