WebP/Lossy: Allow negative values from segment adjustment too

The spec doesn't talk about this happening in the text, but
`dequant_init()` in 20.4 processes segment adjustment and quantization
index adjustment in the same variable `q` before clamping.
Since we had to adjust the latter step in the previous commit, do
it for the former step too.

I haven't seen this happen in the wild yet (and now, I hopefully
never will notice it if it happens).

1 files changed, 1 insertions, 1 deletions

diff --git a/Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp b/Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp
index fe4c0a726f..ec05396cb3 100644
--- a/Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp
+++ b/Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp
@@ -613,7 +613,7 @@ i16 dequantize_value(i16 value, bool is_dc, QuantizationIndices const& quantizat
     // * For uv, the dc_qlookup index is clamped to 117 (instead of 127 for everything else)
     //   (or, alternatively, the value is clamped to 132 at most)
 
-    u8 y_ac_base = quantization_indices.y_ac;
+    int y_ac_base = quantization_indices.y_ac;
     if (segmentation.update_macroblock_segmentation_map) {
         if (segmentation.segment_feature_mode == SegmentFeatureMode::DeltaValueMode)
             y_ac_base += segmentation.quantizer_update_value[segment_id];