Put a glyph's offset on the grid where it is placed

A placed glyph's offset is whole pixels by construction -- a floored pen
position plus the entry's integer bearing -- and `Painter::glyphs` was
converting it, and the entry's width and height, from `f32` on every frame
that drew the glyph. It is a `PxVec2` now, converted once when the text is
placed, and the size is two integer shifts.

Measured with `perf stat -e instructions:u`, since the difference is smaller
than this machine's clock: the `many` phase went from 2,013,099,594
instructions to 1,938,264,572 over 500 frames, 3.7% less. `scroll` and
`repaint` are unchanged to within noise, which is right -- they do not redraw
glyphs.

Checked: fmt, clippy, 105 tests, the reorder fuzzer at 300 seeds, and `tabs`,
`text` and `random` byte-identical.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/primitive/text.rs

@@ -1,7 +1,8 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, TimerKind};
 use crate::{
-    Align, GlyphAtlas, GlyphEntry, GlyphKey, PlacedGlyph, RegionAlign, UiColor, util::Vec2,
+    Align, GlyphAtlas, GlyphEntry, GlyphKey, PlacedGlyph, Px, PxVec2, RegionAlign, UiColor,
+    util::Vec2,
 };
 use parley::{
     Alignment, AlignmentOptions, FontContext, FontFamily, FontFamilyName, GenericFamily, Layout,
@@ -303,9 +304,9 @@ impl TextData {
                     };
                     placed.push(PlacedGlyph {
                         entry,
-                        offset: Vec2::new(
-                            glyph.x.floor() + entry.left as f32,
-                            glyph.y.floor() - entry.top as f32,
+                        offset: PxVec2::new(
+                            Px::from_int(glyph.x.floor() as i32 + entry.left),
+                            Px::from_int(glyph.y.floor() as i32 - entry.top),
                         ),
                     });
                 }