LibGfx/JPEG: Still iterate over AC coefficients of a EOB targeted block

This commit is nonsense for anything else than SOF2 images with spectral
approximation. For this particular case, skips like EOB or ZRL only
apply to coefficients with a zero-history. This commit prepares the code
to handle this behavior.

Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp

@@ -365,6 +365,12 @@ static ErrorOr<bool> read_eob(Scan& scan, u32 symbol)
 
  scan.end_of_bands_run_count = additional_value + (1 << eob_base) - 1;
 
+ if (scan.end_of_bands_run_count >= 1) {
+ // end_of_bands_run_count is decremented at the end of `build_macroblocks`.
+ // This line compensate this effect.
+ ++scan.end_of_bands_run_count;
+ }
+
  return true;
  }
 
@@ -390,6 +396,9 @@ static ErrorOr<void> add_ac(JPEGLoadingContext& context, Macroblock& macroblock,
  auto first_coefficient = max(1, scan.spectral_selection_start);
 
  for (int j = first_coefficient; j <= scan.spectral_selection_end;) {
+ if (scan.end_of_bands_run_count > 0)
+ continue;
+
  // AC symbols encode 2 pieces of information, the high 4 bits represent
  // number of zeroes to be stuffed before reading the coefficient. Low 4
  // bits represent the magnitude of the coefficient.
@@ -462,18 +471,18 @@ static ErrorOr<void> build_macroblocks(JPEGLoadingContext& context, Vector<Macro
  continue;
  }
 
- // G.1.2.2 - Progressive encoding of AC coefficients with Huffman coding
- if (context.current_scan.end_of_bands_run_count > 0) {
- --context.current_scan.end_of_bands_run_count;
- continue;
- }
-
  Macroblock& block = macroblocks[macroblock_index];
 
  if (context.current_scan.spectral_selection_start == 0)
  TRY(add_dc(context, block, scan_component));
  if (context.current_scan.spectral_selection_end != 0)
  TRY(add_ac(context, block, scan_component));
+
+ // G.1.2.2 - Progressive encoding of AC coefficients with Huffman coding
+ if (context.current_scan.end_of_bands_run_count > 0) {
+ --context.current_scan.end_of_bands_run_count;
+ continue;
+ }
  }
  }
  }