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/GIFLoader.cpp

@@ -614,12 +614,23 @@ bool GIFImageDecoderPlugin::set_nonvolatile(bool& was_purged)
     return m_context->frame_buffer->set_nonvolatile(was_purged);
 }
 
-bool GIFImageDecoderPlugin::sniff()
+bool GIFImageDecoderPlugin::initialize()
 {
     InputMemoryStream stream { { m_context->data, m_context->data_size } };
     return !decode_gif_header(stream).is_error();
 }
 
+ErrorOr<bool> GIFImageDecoderPlugin::sniff(ReadonlyBytes data)
+{
+    InputMemoryStream stream { { data.data(), data.size() } };
+    return !decode_gif_header(stream).is_error();
+}
+
+ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> GIFImageDecoderPlugin::create(ReadonlyBytes data)
+{
+    return adopt_nonnull_own_or_enomem(new (nothrow) GIFImageDecoderPlugin(data.data(), data.size()));
+}
+
 bool GIFImageDecoderPlugin::is_animated()
 {
     if (m_context->error_state != GIFLoadingContext::ErrorState::NoError) {