Previously VPB tiles were loaded by the STG file loader.
This was not ideal as the VPB tile granularity is 1x1 degree
while the STG file loader is 20x20km.
This change makes the tile manager load VPB tiles explicitly
on range, and allows better prioritization of the underlying
terrain.
This way of scenery paging is really application code.
Now that the simgear stg loader is seperated from the paging
code, this appication specific paging can reside here.
May be at some time also use the spt stuff here.
structure. The new approach is simpler, more flexible, and more dynamics.
We can now dynamically size the tile cache up and down. Also, the range
of tiles to load is now dependent on visibility and is calculated to always
bring in enough tiles.
from the pre-ssg / render everything ourselves days. Replaced with a
material library manager that is much better suited for working in the
context of ssg. This simplified and cleaned up a ton of old junk.
