Add premultiplied option for raw pixel exports

[ttarczewski]

Currently getImageData, toBuffer('raw'), and toBufferSync('raw') all return RGBA pixel data in unpremultiplied (straight alpha) form, consistent with the HTML Canvas specification. Although the underlying canvas bitmap is typically stored in premultiplied form, every read-back path performs automatic unpremultiplication, and there is no API for retrieving the premultiplied pixel data directly.

Why this matters

• Premultiplied alpha is commonly preferred for GPU upload paths because it matches the representation used by GPU blending pipelines. Applications therefore typically need to re-premultiply the data during upload. This introduces unnecessary overhead when the original pixel data was already premultiplied before skia-canvas unpremultiplied it during read-back.
• The premultiplied-unpremultiplied-premultiplied round-trip is lossy at low alpha values, since unpremultiplication quantizes color channels.

Implementation

Add a new boolean option, premultiplied (defaulting to false). When enabled, the ImageInfo is constructed with AlphaType::Premul instead of AlphaType::Unpremul, bypassing unpremultiplication and returning the underlying premultiplied pixel data directly.