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/PNGLoader.cpp

@@ -834,4 +834,27 @@ bool PNGImageDecoderPlugin::sniff()
     return decode_png_header(*m_context);
 }
 
+bool PNGImageDecoderPlugin::is_animated()
+{
+    return false;
+}
+
+size_t PNGImageDecoderPlugin::loop_count()
+{
+    return 0;
+}
+
+size_t PNGImageDecoderPlugin::frame_count()
+{
+    return 1;
+}
+
+ImageFrameDescriptor PNGImageDecoderPlugin::frame(size_t i)
+{
+    if (i > 0) {
+        return { bitmap(), 0 };
+    }
+    return {};
+}
+
 }