LibGfx: Re-structure the whole initialization pattern for image decoders

When trying to figure out the correct implementation, we now have a very
strong distinction on plugins that are well suited for sniffing, and
plugins that need a MIME type to be chosen.

Instead of having multiple calls to non-static virtual sniff methods for
each Image decoding plugin, we have 2 static methods for each
implementation:
1. The sniff method, which in contrast to the old method, gets a
    ReadonlyBytes parameter and ensures we can figure out the result
    with zero heap allocations for most implementations.
2. The create method, which just creates a new instance so we don't
    expose the constructor to everyone anymore.

In addition to that, we have a new virtual method called initialize,
which has a per-implementation initialization pattern to actually ensure
each implementation can construct a decoder object, and then have a
correct context being applied to it for the actual decoding.

Userland/Libraries/LibGfx/PNGLoader.cpp

@@ -913,11 +913,24 @@ bool PNGImageDecoderPlugin::set_nonvolatile(bool& was_purged)
     return m_context->bitmap->set_nonvolatile(was_purged);
 }
 
-bool PNGImageDecoderPlugin::sniff()
+bool PNGImageDecoderPlugin::initialize()
 {
     return decode_png_header(*m_context);
 }
 
+ErrorOr<bool> PNGImageDecoderPlugin::sniff(ReadonlyBytes data)
+{
+    PNGLoadingContext context;
+    context.data = data.data();
+    context.data_size = data.size();
+    return decode_png_header(context);
+}
+
+ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> PNGImageDecoderPlugin::create(ReadonlyBytes data)
+{
+    return adopt_nonnull_own_or_enomem(new (nothrow) PNGImageDecoderPlugin(data.data(), data.size()));
+}
+
 bool PNGImageDecoderPlugin::is_animated()
 {
     return false;