Preserve unparseable TTML/SRT color values instead of dropping them

[jason-valenzuela]

Problem

newColorFromHTMLString decoded only 6-digit #RRGGBB and 8 named colors. For any other value it returned an error, and both callers — the TTML reader (ttml.go) and the SRT reader (srt.go) — gate on err == nil, so the color was silently discarded on read and omitted on write.

That dropped values which are valid per the specs:

• #RRGGBBAA (8-digit, with alpha) — valid per TTML1 §8.3.2 and the form IMSC1 mandates for tts:color.
• Named colors outside the 8 recognized (e.g. silver, gray, maroon, transparent) — part of the TTML1 §8.3.2 <namedColor> set.
• rgb() / rgba() functional notation.

A TTML in → TTML out (or SRT) round-trip therefore lost styling the source explicitly set.

Fix

• Add an unexported Color.raw field holding the original expression when it can't be decoded into an RGBA triple; HTMLString() returns it verbatim. So #RRGGBBAA, rgb()/rgba(), transparent, and unrecognized names now round-trip losslessly instead of being dropped.
• Decode the rest of the TTML1 named-color set (silver, gray, maroon, purple, fuchsia, lime, olive, navy, teal, aqua) into RGBA, reusing the existing Color* vars, alongside the 8 already handled.
• newColorFromHTMLString can no longer fail, so its now-dead error return is removed and the three call sites simplified to a direct assignment. An empty/blank expression yields a nil color (attribute stays omitted) rather than being flattened to #000000.

Decoded colors still normalize to hex on write (e.g. white → #ffffff, silver → #c0c0c0), which was already the case for the original 8 and is valid output.

Test

TestColorHTMLRoundTrip covers #RRGGBB and named-color decoding, verbatim round-trip of #RRGGBBAA / transparent / an unrecognized name / rgb(), and empty/blank → nil. gofmt, go build ./..., and go test ./... all pass.

Notes

Scope is intentionally minimal: #RRGGBBAA, rgb()/rgba(), and transparent are preserved verbatim rather than decoded into RGBA. Decoding #RRGGBBAA specifically would need Color to distinguish "alpha absent" from "alpha == 0x00" (the struct's Alpha uint8 defaults to 0), which ripples into SSAString/WebVTTString; that's a larger change best left as a follow-up.