frame() still returns a regular RGB Bitmap (now lazily converted
from internal CMYK data), but JPEGImageDecoderPlugin now also
implements cmyk_frame().
This is a hack: Ideally we'd have a CMYK Bitmap pixel format,
and we'd convert to rgb at blit time. Then we could also apply color
profiles (which for CMYK images are CMYK-based).
Also, the colors for our CMYK->RGB conversion are off for PDFs,
and we have distinct codepaths for this in Gfx::Color (for paths)
and JPEGs. So when we fix that, we'll have to fix it in two places.
But this doesn't require a lot of code and it's a huge visual
progression, so let's go with it for now.
The ideal size is the size the user will display the image. Raster
formats should ignore this parameter, but vector formats can use
it to generate a bitmap of the ideal size.
