Use offsetX/offsetY to compute mouse position in default raycaster

[axelboc]

Fix #350, including the regression involving a scrollable container on top of the canvas that receives R3F's event handlers - cf. 42569b9.

As discussed, I add the ability for the user to pass a computeOffsets function to compute the correct offsetX and offsetY values to be used in the raycaster based on the scroll position:

const computeOffsets = useCallback((event) => {
  const { offsetX, offsetY } = event.nativeEvent
  return {
    offsetX,
    offsetY: offsetY - state.scrollTop,
  }
}, [])

<Canvas
  // ...
  raycaster={{ computeOffsets }}
/>

[drcmda]

thanks! seems to work: https://codesandbox.io/s/r3flex-forked-8trbt it's published, too