[gpos] Keep GPOS variation-index deltas fractional

[behdad]

Fixes #402

Problem

GPOS Device/VariationIndex deltas — both value-record adjustments and anchor
coordinates — were read via ItemVariationStore::compute_delta, which rounds
the delta to whole font units, and then scaled to output units. That double
rounding discards up to half a font unit of the advance / placement /
attachment adjustment.

HarfBuzz instead keeps the delta fractional and rounds only once, after scaling
to output units (Device::get_x_delta is em_scalef_x(get_delta()), i.e.
roundf(delta * scale / upem), used for both value records and
Anchor::get_anchor). For variable fonts this made the harfrust shaper
disagree with ot by sub-font-unit amounts on non-default instances — e.g.
SFNSItalic.ttf shaping "System italic font at weight N" at weights other
than the default (#402).

Fix

Read the deltas with compute_float_delta and scale them with new
Scale::scale_x_f / scale_y_f helpers that mirror HarfBuzz's em_multf
(roundf(v * scale / upem)), keeping f32 arithmetic to match HarfBuzz. The
static value-record path stays on the integer scale, matching HarfBuzz
(static → em_scale, device → em_scalef). resolve_anchor now returns
fractional font units, and its cursive and mark callers round once when
scaling.

Requires read-fonts 0.41.0

This bumps the read-fonts dependency to 0.41.0 for the public
FloatItemDelta::to_f64 accessor (googlefonts/fontations#1954). CI will build
once read-fonts 0.41.0 is published on crates.io.

Testing

• SFNSItalic.ttf advances now match the ot shaper exactly (0.000000px):
  hb-shape --shaper harfrust vs --shaper ot is byte-identical across
  weights 100–900.
• cargo test -p harfrust: 6027 passed, 0 failed.
• HarfBuzz meson test --suite shape: 3/3 OK (8912 subtests), 0 failures.