wrlib: new headers to contain definition of internal stuff

Using local function prototype redefinition is dangerous, now a single
prototype is seen in all the files that are concerned.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

wrlib/convert.c

@@ -35,12 +35,9 @@
 
 #include "wraster.h"
 #include "convert.h"
+#include "xutil.h"
 
 
-#ifdef USE_XSHM
-extern Pixmap R_CreateXImageMappedPixmap(RContext * context, RXImage * ximage);
-#endif
-
 #define NFREE(n)  if (n) free(n)
 
 #define HAS_ALPHA(I)	((I)->format == RRGBAFormat)