WebP/Lossy: Clamp negative quantization indices to zero

The spec doesn't talk about this happening in the text, but
`dequant_init()` in 20.4 stores `q` in an int and clamps that
to 0 later.

Tests/LibGfx/TestImageDecoder.cpp

@@ -404,6 +404,27 @@ TEST_CASE(test_webp_extended_lossy_uncompressed_alpha)
     EXPECT_EQ(frame.image->get_pixel(355, 106), Gfx::Color(0, 0, 0, 0));
 }
 
+TEST_CASE(test_webp_extended_lossy_negative_quantization_offset)
+{
+    auto file = MUST(Core::MappedFile::map(TEST_INPUT("smolkling.webp"sv)));
+    EXPECT(Gfx::WebPImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = MUST(Gfx::WebPImageDecoderPlugin::create(file->bytes()));
+    MUST(plugin_decoder->initialize());
+
+    EXPECT_EQ(plugin_decoder->frame_count(), 1u);
+    EXPECT(!plugin_decoder->is_animated());
+    EXPECT(!plugin_decoder->loop_count());
+
+    EXPECT_EQ(plugin_decoder->size(), Gfx::IntSize(264, 264));
+
+    auto frame = MUST(plugin_decoder->frame(0));
+    EXPECT_EQ(frame.image->size(), Gfx::IntSize(264, 264));
+
+    // While VP8 YUV contents are defined bit-exact, the YUV->RGB conversion isn't.
+    // So pixels changing by 1 or so below is fine if you change code.
+    EXPECT_EQ(frame.image->get_pixel(16, 16), Gfx::Color(0x3c, 0x24, 0x1a, 255));
+}
+
 TEST_CASE(test_webp_lossy_4)
 {
     // This is https://commons.wikimedia.org/wiki/File:Fr%C3%BChling_bl%C3%BChender_Kirschenbaum.jpg,

Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp

@@ -621,7 +621,7 @@ i16 dequantize_value(i16 value, bool is_dc, QuantizationIndices const& quantizat
             y_ac_base = segmentation.quantizer_update_value[segment_id];
     }
 
-    u8 dequantization_index;
+    int dequantization_index;
     if (index.is_y2())
         dequantization_index = y_ac_base + (is_dc ? quantization_indices.y2_dc_delta : quantization_indices.y2_ac_delta);
     else if (index.is_u() || index.is_v())
@@ -631,9 +631,9 @@ i16 dequantize_value(i16 value, bool is_dc, QuantizationIndices const& quantizat
 
     // clamp index
     if ((index.is_u() || index.is_v()) && is_dc)
-        dequantization_index = min(dequantization_index, 117);
+        dequantization_index = clamp(dequantization_index, 0, 117);
     else
-        dequantization_index = min(dequantization_index, 127);
+        dequantization_index = clamp(dequantization_index, 0, 127);
 
     // "the multiplies are computed and stored using 16-bit signed integers."
     i16 dequantization_factor;