LibGfx: Allow images to report that they are originally grayscale

...and implement it in JPEGLoader.

Since it's easy to get the grayscale data off a Bitmap, don't
add a grayscale_frame() accessor.

Userland/Libraries/LibGfx/ImageFormats/ImageDecoder.h

@@ -31,6 +31,7 @@ struct VectorImageFrameDescriptor {
 
 enum class NaturalFrameFormat {
     RGB,
+    Grayscale,
     CMYK,
     Vector,
 };