UI: resize some entity graphics
UI: use specific asteroid mesh if specified

Add TODO around async resource loading order
UI: add callback to scale entity size with camera distance

If overridden entities can customize size / appearance of
entity gfx w/ camera distance.

Also use static position / size scale when in far scene mode,
other scene modes use sliding scale as before.
UI: scale planet mesh/axis & ship/station indicators w/ cam dist
UI: tweak camera settings
UI: add helper to force camera update

Will trigger event handlers / callbacks even if camera properties
haven't been changed.

Also small tweak to Canvas#is_following to genericize following
check if no test component is specified