UI materials: don't reserve in loop when enough capacity

[mockersf]

Objective

• UI materials reserve too much capacity in a vec: for every node in the transparent phase, it reserves enough memory to store all the nodes
• Update UI Materials: don't reserve items on phase from inside the loop #10437

Solution

• Only reserve extra memory if there's not enough
• Only reserve the needed memory, not more

[ickshonpe]

The transparent phase items vec is shared between all the extracted UI items, not just uimaterial nodes, so this isn't quite right either. Maybe the current length of items needs to be stored before the start of the loop, then this condition could become transparent_phase.items.capacity() - initial_len < extracted.uinodes.len()?

[mockersf]

so it seems we have no way to know the exact size required on each transparent phase, as not all nodes may go to the same if there are several cameras

Current behaviour on main is that for each entity, it's reserving "number of entities" capacity, so this PR is still an improvement
instead of:

• for each entity
  • ensure phase can store "number of entities" more items <- allocate for every pass through the loop
  • push to the phase

it does:

• for each entity
  • ensure once that phase can store "number of entities" more items <- allocate once
  • push to the phase <- allocate once capacity has been reached for every pass through the loop

[ickshonpe]

Yeah that makes sense. We can merge this and look for further improvements later.