LibGfx/JBIG2: Call decode_immediate_generic_region for lossless regions

It seems to do the right thing already, and nothing in the spec says
not to do this as far as I can tell.

With this, we can finally decode
Tests/LibGfx/test-inputs/jbig2/bitmap.jbig2 and add a test for
decoding simple arithmetic-coded images.

Tests/LibGfx/TestImageDecoder.cpp

@@ -336,6 +336,23 @@ TEST_CASE(test_jbig2_white_47x23)
         EXPECT_EQ(pixel, Gfx::Color(Gfx::Color::White).value());
 }
 
+TEST_CASE(test_jbig2_generic_region_arithmetic_code)
+{
+    auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("jbig2/bitmap.jbig2"sv)));
+    EXPECT(Gfx::JBIG2ImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = TRY_OR_FAIL(Gfx::JBIG2ImageDecoderPlugin::create(file->bytes()));
+
+    auto bmp_file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("bmp/bitmap.bmp"sv)));
+    auto bmp_plugin_decoder = TRY_OR_FAIL(Gfx::JBIG2ImageDecoderPlugin::create(file->bytes()));
+
+    auto frame = TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 399, 400 }));
+    auto bmp_frame = TRY_OR_FAIL(expect_single_frame_of_size(*bmp_plugin_decoder, { 399, 400 }));
+
+    for (int y = 0; y < frame.image->height(); ++y)
+        for (int x = 0; x < frame.image->width(); ++x)
+            EXPECT_EQ(frame.image->get_pixel(x, y), bmp_frame.image->get_pixel(x, y));
+}
+
 TEST_CASE(test_jbig2_arithmetic_decoder)
 {
     // https://www.itu.int/rec/T-REC-T.88-201808-I

Userland/Libraries/LibGfx/ImageFormats/JBIG2Loader.cpp

@@ -922,11 +922,6 @@ static ErrorOr<void> decode_immediate_generic_region(JBIG2LoadingContext& contex
     return {};
 }
 
-static ErrorOr<void> decode_immediate_lossless_generic_region(JBIG2LoadingContext&, SegmentData const&)
-{
-    return Error::from_string_literal("JBIG2ImageDecoderPlugin: Cannot decode immediate lossless generic region yet");
-}
-
 static ErrorOr<void> decode_intermediate_generic_refinement_region(JBIG2LoadingContext&, SegmentData const&)
 {
     return Error::from_string_literal("JBIG2ImageDecoderPlugin: Cannot decode intermediate generic refinement region yet");
@@ -1118,10 +1113,13 @@ static ErrorOr<void> decode_data(JBIG2LoadingContext& context)
             TRY(decode_intermediate_generic_region(context, segment));
             break;
         case SegmentType::ImmediateGenericRegion:
-            TRY(decode_immediate_generic_region(context, segment));
-            break;
         case SegmentType::ImmediateLosslessGenericRegion:
-            TRY(decode_immediate_lossless_generic_region(context, segment));
+            // 7.4.6 Generic region segment syntax
+            // "The data parts of all three of the generic region segment types ("intermediate generic region", "immediate generic region" and
+            //  "immediate lossless generic region") are coded identically, but are acted upon differently, see 8.2."
+            // But 8.2 only describes a difference between intermediate and immediate regions as far as I can tell,
+            // and calling the immediate generic region handler for immediate generic lossless regions seems to do the right thing (?).
+            TRY(decode_immediate_generic_region(context, segment));
             break;
         case SegmentType::IntermediateGenericRefinementRegion:
             TRY(decode_intermediate_generic_refinement_region(context, segment));