Make text in PDF exports extractable

[Mythie]

Text drawn with fillText() isn't copy/paste-able or searchable in exported PDFs when the shaper substitutes glyphs with no cmap entry (ligatures, stylistic sets):

FontLibrary.use('Caveat', 'Caveat.ttf')
ctx.font = '36px Caveat'
ctx.fillText('Lucas Smithy Bruz', 20, 60)
await canvas.saveAs('out.pdf')  // pdftotext → "Luc  t  Bru"

Paragraph::paint emits bare text blobs, so SkPDF builds /ToUnicode by reverse-cmap lookup and maps substituted glyphs to <0000>.

The fix draws the laid-out runs via drawGlyphs with utf8 & cluster info attached, which SkPDF turns into correct mappings and /ActualText spans.

Rendered output is unchanged. For decorated text, the glyphs are drawn once by the new code path, then Paragraph::paint is still called so skparagraph draws its underline/overline/strike-through lines. Its own glyph drawing is disabled (via a no-op blend mode) so the text isn't painted twice.

The second commit fixes textDecoration ignoring any value without an explicit color (currentColor was treated as invalid). Happy to drop this commit though since it isn't really relevant to what I'm trying to solve.

Both commits add tests that fail on main. Wrapped, RTL, and ligature text verified with pdftotext.