Factor out a single PNG-reading function to encapsulate the libpng API (#1765)

include/gfx/png.hpp

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+
+#ifndef RGBDS_GFX_PNG_HPP
+#define RGBDS_GFX_PNG_HPP
+
+#include <fstream>
+#include <stdint.h>
+#include <vector>
+
+#include "gfx/rgba.hpp"
+
+struct Png {
+	uint32_t width, height;
+	std::vector<Rgba> pixels{};
+	std::vector<Rgba> palette{};
+
+	Png() {}
+	Png(char const *filename, std::streambuf &file);
+};
+
+#endif // RGBDS_GFX_PNG_HPP