Respect explicit BASIC layout in font_variant()

[iAlturki]

FreeTypeFont.font_variant(layout_engine=ImageFont.Layout.BASIC) silently keeps
the original font's Raqm engine. Layout.BASIC has the value zero, so the current
layout_engine or self.layout_engine expression discards the explicit override.
This changes shaping, kerning and text measurements despite the requested engine.

Changes proposed in this pull request:

• Inherit the original layout engine only when the argument is None, consistent
  with the other overrides in font_variant().
• Test all four original/requested engine combinations, compare text measurement
  with a directly constructed font, and check that an omitted engine is inherited.
• Add a release note.

from PIL import ImageFont

font = ImageFont.truetype(
    "Tests/fonts/DejaVuSans/DejaVuSans.ttf",
    54,
    layout_engine=ImageFont.Layout.RAQM,
)
variant = font.font_variant(layout_engine=ImageFont.Layout.BASIC)
print(variant.layout_engine.name)  # Before: RAQM; after: BASIC

Measurements and reproduction script

The comparison uses generated text and the checkout's bundled DejaVu Sans font.
With FreeType 2.13.2 and Raqm 0.10.1, the copied font's advance for AVATAR office
changes from 368.28125 px to the requested BASIC engine's 385 px. Its rendering
then matches a directly created BASIC font pixel for pixel. The proof files are
on a separate fork branch; this PR changes only the font implementation, tests
and release note.

Validation on Ubuntu 24.04, Python 3.12.3, built from
27af434ef326fd5f71e600ba39c86c5706526fc3 with FreeType and Raqm enabled:

• New engine-override cases against unchanged source: 1 failed, 3 passed.
• Fixed font-copy checks: 8 passed.
• python3 selftest.py: 59 passed.
• Full suite (python3 -m pytest Tests/ -n 4 -q): 5,495 passed, 220 skipped,
  3 xfailed, no failures. Skipped cases include optional codecs, GUI bindings and platform-specific tests.
• All pinned pre-commit hooks, including the manual packaging check, pass.
• Configured tox -e mypy: no issues in 303 source files.

Prepared with AI assistance. Commit author: iAlturki.

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