gpui: Clear the element arena after presenting the frame (#33338)

This is an easy way to shave some microseconds off the critical path for
frame rendering. On my machine this reduces typical frame rendering
latency by ~100 microseconds, probably quite a bit more on slower
machines.

Here is how long it typically takes to drop elements from the arena,
from a fairly brief run:


![image](https://github.com/user-attachments/assets/65cfd911-eccf-4393-887d-8cad2cd27148)

Release Notes:

- N/A
9427526a41Michael Sloan committed on 6/24/2025, 7:43:33 PM· committed by GitHubparenteec26c9
3 files changedLine totals unavailable