Fix handling of PNG files with RGB or RGBA color type that have a PLTE chunk

src/gfx/pal_sorting.cpp

@@ -33,7 +33,13 @@ void sortIndexed(std::vector<Palette> &palettes, std::vector<Rgba> const &embPal
 					return true;
 				}
 			}
-			unreachable_(); // LCOV_EXCL_LINE
+			// A PNG image using PNG_COLOR_TYPE_RGB (2) or PNG_COLOR_TYPE_RGBA (6) can still
+			// contain a PLTE chunk. From the PNG spec: "If present, it provides a suggested set of
+			// from 1 to 256 colors to which the truecolor image can be quantized if the viewer
+			// cannot display truecolor directly." This means that an input image may have pixels
+			// with colors that are not in the PLTE chunk, so iterating through `embPal` may finish
+			// without returning. In that case, we compare `lhs` and `rhs` directly.
+			return lhs < rhs;
 		});
 	}
 }