Use Vec instead of SmallVec for glyphs field of ShapedRun (#30664)

This glyphs field is usually larger than 8 elements, and SmallVec is not
efficient when it cannot store the value inline.

This change also adds precise glyphs run preallocation in some places
`ShapedRun` is constructed.

Release Notes:

- N/A

crates/gpui/src/text_system/line_layout.rs

@@ -34,7 +34,7 @@ pub struct ShapedRun {
     /// The font id for this run
     pub font_id: FontId,
     /// The glyphs that make up this run
-    pub glyphs: SmallVec<[ShapedGlyph; 8]>,
+    pub glyphs: Vec<ShapedGlyph>,
 }
 
 /// A single glyph, ready to paint.