fix(render): respect color alpha channel in render_to_png

[mikhaelmartin]

Closes #103

render_to_png discards the alpha channel from Color values in all four paint paths. The pipeline correctly flows Color::a through LayoutOptions → DisplayList → renderer, but each paint operation hardcodes alpha to 255:

Function	Line
render_glyph_with_font	569
render_line	705
render_rect	741
render_path_segment	861

This replaces the hardcoded 255 with (color.a * 255.0) as u8, matching how the RGB components already use (color.r * 255.0) as u8 etc.

[techwizrd]

This fixes the hardcoded alpha in the paint paths, but I think PNG encoding still needs attention. tiny-skia::Pixmap stores premultiplied RGBA, and our custom encode_png() writes pixmap.data() directly. With non-opaque foregrounds, that can produce premultiplied RGB in the PNG. Could we use pixmap.encode_png() or demultiply before writing, plus add a foreground alpha regression test?

I'm happy to add a fix if you'd like.

[mikhaelmartin]

@techwizrd Yes, please! I'm pretty new to this, so feel free to push the fix directly to this branch. Thank you so much for the help!