The main goal of this change was to make it easier to tell when a
hierarchy has been deleted, but as a side benefit, it got rid of the use
of PR_RESMAP. Also, it's easy to track the number of hierarchies.
Unfortunately, it showed how brittle the component side of the ECS is
(scene and canvas registries assumed their components were the first (no
long the case), thus the sweeping changes).
Centerprint doesn't work (but it hasn't for a while).
This has use when the order of components in the pool affects draw order
(or has other significance), especially at the subpool level. I plan to
use it for fixing overlapping windows in imui.
I found I needed the subrange start as well as the end, but I liked that
the subpools themselves used only the end of the range, so switching to
just a unint32_t for the value and adding a function to return a tuple
made sense. I had kept the struct because I thought I might want to
store additional information (eg, the entity "owning" the subpool), but
found that I didn't need such information as the systems using subpools
that way would have access to the entity by other means.
Interestingly, the change found a bug in subpool creation: I really
don't know why things worked before, but they work better now :)
Subpools are for grouping components by some criterion. Any component
that has a rangeid callback will be grouped with other components that
return the same render id. Note that the ordering of components within a
group will be affected by adding a component into a group that comes
before that group (or removing a component).
Component pools can have multiple groups, added and removed dynamically,
but removing a group should (currently) be done only when empty.
