assemble a splash preview if none was found (#277)

include/conversion.h

@@ -0,0 +1,9 @@
+#ifndef CONVERISON_H
+#define CONVERISON_H
+
+#include "common.h"
+
+size_t bin_to_abgr(char ** bufp, size_t size);
+size_t png_to_abgr(char ** bufp, size_t size);
+
+#endif