LibGFX/PAM: Allow reading CMYK .pam files

These are written by `mutool extract` for CMYK images.

They don't contain color profiles so they're not super convenient.
But `image` can convert them (*) to sRGB as long as you use it with
`--assign-color-profile` pointing to some CMYK icc profile of your
choice.

*: Once #22922 is merged.

Userland/Libraries/LibGfx/ImageFormats/PAMLoader.h

@@ -18,6 +18,7 @@ struct PAM {
     u16 max_val { 0 };
     u16 depth { 0 };
     String tupl_type {};
+    Optional<NonnullRefPtr<CMYKBitmap>> cmyk_bitmap {};
 };
 
 using PAMLoadingContext = PortableImageMapLoadingContext<PAM>;