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

crates/workspace/src/workspace.rs

@@ -2199,7 +2199,7 @@ impl Workspace {
                             // (Note that the tests always do this implicitly, so you must manually test with something like:
                             //   "bindings": { "g z": ["workspace::SendKeystrokes", ": j <enter> u"]}
                             // )
-                            window.draw(cx);
+                            window.draw(cx).clear();
                         }
                     })?;
                 }