This allows the developer to override them usefully in situations
where they need to be set e.g., make a debug library masquerade as
release.

Also allow CESIUM_DEBUG_PREFIX to be empty.

This is all very ad hoc and there will be more extensions that will
have to be handled in the future. This function is currently only used
by Cmpt parsing code, but one could imagine it being generally useful.

Initial commit of the main body of new code. A lot was taken or
inspired from PntsToGltfConverter, which should now be refactored to
use the new common functions.

ENU rotations are not supported yet.

It's required now.

Construct an RTC_CENTER, in effect. Otherwise instance positions could
be too large to render without jitter.

Simplified the code by not trying to optimize the inclusion of
rotation and scale transforms.

Tests compile again.

Fix a real truncation issue, and some noise from overzealous gcc.

A simple function for computing a perpendicular vector.

Rename and rework arguments in response to review comments.

This follows the change in ImplicitQuadtreeLoader.