WRaster: Bugfix, added missing 'const' to RLoad* functions

Prototype of function changed in commit
  d1e1c13fa3
but two of them were not updated in the source.

Thanks to Rodolfo García Peñas for taking the time to test and report.

wrlib/gif.c

@@ -39,7 +39,7 @@ static int InterlacedJumps[] = { 8, 8, 4, 2 };
 /*
  * Partially based on code in gif2rgb from giflib, by Gershon Elber.
  */
-RImage *RLoadGIF(char *file, int index)
+RImage *RLoadGIF(const char *file, int index)
 {
 	RImage *image = NULL;
 	unsigned char *cptr;

wrlib/nxpm.c

@@ -228,7 +228,7 @@ RImage *RGetImageFromXPMData(RContext * context, char **data)
 	return NULL;
 }
 
-RImage *RLoadXPM(RContext * context, char *file)
+RImage *RLoadXPM(RContext * context, const char *file)
 {
 	RImage *image = NULL;
 	char line[LINEWIDTH + 1];