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/DDSLoader.cpp

@@ -968,7 +968,7 @@ bool DDSImageDecoderPlugin::set_nonvolatile(bool& was_purged)
     return m_context->bitmap->set_nonvolatile(was_purged);
 }
 
-bool DDSImageDecoderPlugin::sniff()
+bool DDSImageDecoderPlugin::initialize()
 {
     // The header is always at least 128 bytes, so if the file is smaller, it can't be a DDS.
     return m_context->data_size > 128
@@ -978,6 +978,21 @@ bool DDSImageDecoderPlugin::sniff()
         && m_context->data[3] == 0x20;
 }
 
+ErrorOr<bool> DDSImageDecoderPlugin::sniff(ReadonlyBytes data)
+{
+    // The header is always at least 128 bytes, so if the file is smaller, it can't be a DDS.
+    return data.size() > 128
+        && data.data()[0] == 0x44
+        && data.data()[1] == 0x44
+        && data.data()[2] == 0x53
+        && data.data()[3] == 0x20;
+}
+
+ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> DDSImageDecoderPlugin::create(ReadonlyBytes data)
+{
+    return adopt_nonnull_own_or_enomem(new (nothrow) DDSImageDecoderPlugin(data.data(), data.size()));
+}
+
 bool DDSImageDecoderPlugin::is_animated()
 {
     return false;