Remove progname variable

The error message shouldn't specify the name of the binary, that's
supposed to be known by the caller.

Update test reference outputs.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/gfx/main.c

@@ -19,8 +19,6 @@
 #include <string.h>
 #include "gfx/main.h"
 
-char *progname;
-
 static void
 usage(void)
 {
@@ -41,8 +39,6 @@ main(int argc, char *argv[])
 	char *ext;
 	const char *errmsg = "Warning: The PNG's %s setting is not the same as the setting defined on the command line.";
 
-	progname = "rgbgfx";
-
 	if (argc == 1) {
 		usage();
 	}