Proposed fix for 5411

[diarmidmackenzie]

Description:

See #5411

Changes proposed:

In VR mode, when no mouse is available, prevent cursor from firing a "mouseenter" event, as follows:

• on entering VR, set a flag this.mouseAvailable to false
• on detection of a mouse move or click event, set this.mouseAvailable to true
• use this.mouseAvailable to determine whether ot not to so an early return on detection of a raycaster intersection.

This is intended to allow use of a mouse in "VR mode" for e.g. cases such as desktop full screen.

This PR is still a draft. I haven't yet:

• run UTs
• added UTs
• done any live testing

If a fix along these lines is wanted, I'll take care of the above, and submit for review. For now awaiting feedback either here or in #5411.