Render v6 explicit RGBA stroke colours

[reyna-b]

Summary

Fix SVG/PDF rendering for reMarkable v6 explicit RGBA stroke colours parsed by rmscene as Line.color_rgba.

Previously, explicit RGBA tuples such as (48, 74, 224, 77) were emitted directly as CSS rgba(48, 74, 224, 77). CSS alpha is not an 8-bit channel, so those strokes render incorrectly. This patch separates colour and opacity:

• stroke colour: rgb(r, g, b)
• opacity: a / 255

It also avoids applying an extra hard-coded fade to v6 SHADER strokes because the file already carries opacity in color_rgba.

Verification

• Added unit coverage for svg_rgb, svg_alpha, normal pen opacity, and shader opacity.
• Ran tests in a clean checkout with the patch applied:

4 passed in 0.10s

• Rendered a real Paper Pro v6 .rm sample containing shader colours and confirmed generated SVG now contains valid CSS like:

stroke:rgb(48, 74, 224); opacity:0.30196078431372547

instead of invalid 8-bit alpha in rgba(...).