LibGfx/PortableFormat: Simplify the State enum

This enum used to store very precise state about the decoding process,
let's simplify that by only including two steps: HeaderDecoder and
BitmapDecoded.

Userland/Libraries/LibGfx/ImageFormats/PPMLoader.cpp

@@ -45,7 +45,7 @@ ErrorOr<void> read_image_data(PPMLoadingContext& context)
         }
     }
 
-    context.state = PPMLoadingContext::State::Bitmap;
+    context.state = PPMLoadingContext::State::BitmapDecoded;
     return {};
 }
 }