Instancing billboards

[lilleyse]

This is merged with the billboard size-in-meters changes.

For #3001

[pjcozzi]

In Firefox, I get six test failures in master, and 22 in this branch. The new ones are BillboardCollection and friends.

[pjcozzi]

The vast majority of this code has already been reviewed and is in the instancing-ready-for-master branch (merged in #3048). This pull request is to bring those changes into master along with fixing merge conflicts from #3043 (billboard size in meters).

[pjcozzi]

@lilleyse please update the task list in #3001 when this is merged (and in general as needed).

[pjcozzi]

This is ready to merge once the test failures in Firefox are fixed.

[lilleyse]

This should fix the errors.

[pjcozzi]

Looks good, thanks!