Only sort generated palette colors by PLTE order for indexed PNG images

Non-indexed images can still have a PLTE chunk. We should use
`sortRgb` instead of `sortIndexed` for them, since their pixels' colors
may not all be present in the PLTE chunk and would be unsortable.

include/gfx/png.hpp

@@ -14,6 +14,7 @@ struct Png {
 	uint32_t height = 0;
 	std::vector<Rgba> pixels{};
 	std::vector<Rgba> palette{};
+	bool isIndexed = false;
 
 	Png() {}
 	Png(char const *filename, std::streambuf &file);