LibGfx: Add support for animated images to ImageDecoder{Plugin}

Adds methods to determine whether an image is animated, how many times
the animation loops, the number of frames, and to get individual frames.

Implements stubs of these methods for PNGImageDecoderPlugin and
GIFImageDecoderPlugin.

Libraries/LibGfx/GIFLoader.cpp

@@ -528,4 +528,28 @@ bool GIFImageDecoderPlugin::sniff()
     BufferStream stream(buffer);
     return decode_gif_header(stream).has_value();
 }
+
+bool GIFImageDecoderPlugin::is_animated()
+{
+    return false;
+}
+
+size_t GIFImageDecoderPlugin::loop_count()
+{
+    return 0;
+}
+
+size_t GIFImageDecoderPlugin::frame_count()
+{
+    return 1;
+}
+
+ImageFrameDescriptor GIFImageDecoderPlugin::frame(size_t i)
+{
+    if (i > 0) {
+        return { bitmap(), 0 };
+    }
+    return {};
+}
+
 }