Fix false positive in googlefonts/colorfont_tables for variable fonts with non-varying SVG glyphs

[Sanjays2402]

Description

Fixes #4946.

The googlefonts/colorfont_tables check unconditionally emitted a
variable-svg FAIL for every variable font containing an SVG table, on the
grounds that SVG does not support OpenType Variations. But not every glyph
in a variable font varies: a variable font whose SVG color glyphs have no
variation data renders identically at every point in the design space, so
flagging it is a false positive (e.g. aliftype/raqq v2.00).

The check now inspects the SVG table's document list and only emits
variable-svg when at least one of the covered color glyphs actually has
variation data — in gvar, or via blend operators in CFF2 charstrings
(including called subroutines). Fonts with a degenerate/empty SVG table
keep the previous conservative behavior. The check rationale was updated
to describe this.

Test evidence

• Reproduced the bug with the font linked in the issue (Raqq v2.00):
  before the fix it got FAIL variable-svg; after the fix it no longer
  does (45 SVG color glyphs, none with variation data).
• Extended tests/test_checks_googlefonts_colorfont_tables.py:
  • a variable font with an SVG table covering glyphs with no gvar
    variation data now PASSes;
  • a variable font whose SVG-covered glyph does have gvar variation
    data still FAILs with variable-svg;
  • a new CFF2 test using source-sans-pro/VAR/SourceSansVariable-Italic.otf:
    SVG covering space (no blend ops) does not FAIL, SVG covering A
    (has blend ops) FAILs with variable-svg.
• pytest tests/test_checks_googlefonts_colorfont_tables.py: 2 passed.
• black --check clean on both changed files; pylint clean on the check module.
• Updated CHANGELOG.md.