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/TGALoader.cpp

@@ -212,11 +212,28 @@ bool TGAImageDecoderPlugin::decode_tga_header()
     return true;
 }
 
-bool TGAImageDecoderPlugin::sniff()
+bool TGAImageDecoderPlugin::initialize()
 {
     return decode_tga_header();
 }
 
+ErrorOr<bool> TGAImageDecoderPlugin::validate_before_create(ReadonlyBytes data)
+{
+    if (data.size() < sizeof(TGAHeader))
+        return false;
+    TGAHeader const& header = *reinterpret_cast<TGAHeader const*>(data.data());
+    if (header.data_type_code == TGADataType::UncompressedRGB && data.size() < (header.width * header.height * (header.bits_per_pixel / 8)))
+        return false;
+    if (header.bits_per_pixel < 8 || header.bits_per_pixel > 32)
+        return false;
+    return true;
+}
+
+ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> TGAImageDecoderPlugin::create(ReadonlyBytes data)
+{
+    return adopt_nonnull_own_or_enomem(new (nothrow) TGAImageDecoderPlugin(data.data(), data.size()));
+}
+
 bool TGAImageDecoderPlugin::is_animated()
 {
     return false;