Configurable sortOrder for raycast component

[diarmidmackenzie]

Description:

This PR addresses use cases described here:
#4965

Changes proposed:

• New property on raycast, which allows a user to specify a sortOrder to use, in place of the default distance order.
• This sortOrder is provided in the form of another attribute on the objects, that takes values indicating the desired order.
• I've also had to make a fix to cursor.js to fully trust the order of objects it gets from raycast, rather than making its own decisions based on relative distances. As far as I can tell the code I have removed here was superfluous. However this is probably the biggest risk with the PR in terms of possible regressions.

For more details o how this works see the additions to the documentation, and the example included in the PR.

[diarmidmackenzie]

Made a further fix to cursor.js, because raycaster was not behaving as expected.
#4974

The problem scenario was when a small object was behind a large object of equal priority. The small object was getting selected due to the fact that raycaster was not reporting the intersection with the larger object, since it was not new.