Stop sending " " to linebreaker for replaced content (#30740)

We previously sent a " " to the linebreaker in order to ensure that the
next text had a soft wrap opportunity at the start. Calling `next(" ")`
without waiting until the returned index was 1, violated some
invariants of linebreaker ultimately causing a panic.

Instead of using the linebreaker for this, simply keep a flag in the
IFC layout state, which avoids the problem entirely.

Fixes #30703.

components/gfx/text/text_run.rs

@@ -220,7 +220,7 @@ impl<'a> TextRun {
             if text.len() == 0 {
                 return (glyphs, true);
             }
-            *breaker = Some(LineBreakLeafIter::new(&text, 0));
+            *breaker = Some(LineBreakLeafIter::new(text, 0));
         }
 
         let breaker = breaker.as_mut().unwrap();