Compose SVG glyph transforms in document order

[jafin]

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Description

Fixes #570.

Matrix3x2 transforms row vectors, so in a * b the transform a applies first. SvgGlyphSource composed SVG transforms the other way round. This PR fixes the order in three places, and the rotation centre:

• Transform lists: ParseTransform now prepends each entry (m = t * m), so the list applies right to left as SVG requires.
• Nested elements: Walk now computes nodeTransform * parentLocalTransform.
• <use>: now computes translate(x, y) * nodeTransform * parentLocalTransform. The x/y offset and the element's own transform were already in the right order relative to each other; only the parent transform was misplaced.
• rotate(angle cx cy): now rotates about (cx, cy), using Matrix3x2.CreateRotation(radians, centerPoint).

gradientTransform goes through the same ParseTransform, so a gradient with more than one transform in its list is fixed as well.

Tests

SvgTransformTests renders a triangle under each transform and compares its device-space points with an untransformed triangle at hand-calculated coordinates. Five of the six cases fail on main. The sixth checks the x/y-before-transform order inside <use>, which was already correct.

SvgTestFonts is a small shared helper that adds an SVG table to SixLaborsSampleAB.ttf, so tests can write SVG glyph documents inline. #569 has a private copy of the same helper. Whichever of the two merges second can switch to the shared one.

The full suite passes on net10.0 in Release, apart from HarfBuzzCorpusTests (2 tests) and TextShaperTests.ZawgyiScriptOverrideAppliesThroughTextOptionsAndTextRun, which fail the same way on a clean main in my environment.

I also enabled SUPPORTS_DRAWING locally to compare the Issues_462 emoji snapshots. Their differences from the reference images are identical with and without this change, so it doesn't change the Noto Color Emoji SVG output. (12 of those 16 snapshots differ from the references on main on my machine anyway, the COLRv1 ones included.)

Noticed but not changed

Two more transform issues in the same file, left out to keep this PR focused:

• A transform on the root <svg> element is passed to Walk as the starting transform and is also stored in PaintedCanvasMetadata.RootTransform, which PaintedGlyphMetrics.ComputeSourceToUpem applies again. When the glyph is the root element itself, Walk parses the attribute a third time.
• BuildLinearGradient and BuildRadialGradient multiply gradientTransform together along an href chain. In SVG, a referencing gradient inherits the attribute only when it doesn't set its own.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80%. Comparing base (4eb468d) to head (fe6688a).

Files with missing lines	Patch %	Lines
...xLabors.Fonts/Tables/General/Svg/SvgGlyphSource.cs	66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #571   +/-   ##
=====================================
  Coverage     80%     80%           
=====================================
  Files        408     408           
  Lines      35802   35802           
  Branches    5542    5543    +1     
=====================================
+ Hits       28885   28914   +29     
+ Misses      5488    5448   -40     
- Partials    1429    1440   +11     

Flag	Coverage Δ
unittests	80% <66%> (+<1%)	⬆️

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