LibGfx: Add a sniff method to ImageDecoder and implement for GIF and PNG

The sniff method is intended to be used for content sniffing. It should be a
cheap test to rapidly rule out whether a candidate image can be successfully
decoded. For the GIF and PNG implementations it simply attempts to decode the
image header, returning true if successful and false if not.

Libraries/LibGfx/PNGLoader.cpp

@@ -725,4 +725,9 @@ bool PNGImageDecoderPlugin::set_nonvolatile()
     return m_context->bitmap->set_nonvolatile();
 }
 
+bool PNGImageDecoderPlugin::sniff()
+{
+    return decode_png_header(*m_context);
+}
+
 }