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/orientation/pos.rs

@@ -28,6 +28,14 @@ impl UiVec2 {
         }
     }
 
+    /// From lengths already on the grid, with no fraction of a box.
+    pub const fn from_px(px: PxVec2) -> Self {
+        Self {
+            x: UiScalar::from_parts(Rel::ZERO, px.x),
+            y: UiScalar::from_parts(Rel::ZERO, px.y),
+        }
+    }
+
     pub const fn rel(rel: impl const Into<Vec2>) -> Self {
         let rel = rel.into();
         Self {