editor: Do not lay out task indicators outside of the viewport (#17250)

A friend of mine shared a Rust file with me that crashed Zed
consistently due to Arena space exhaustion. It is a dump of a proc macro
output that generates tests (among other things).
TL;DR: we were always laying out all run indicators, irrespective of
current scroll position. In his case, we were redundantly rendering
about 3k elements.
Obviously, this doesn't just fix the problems with Arena space
exhaustion - it should also improve perf in files with many runnables.

Release Notes:

- Improved editor performance in presence of many runnable indicators in
the gutter.