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.
