Converted #define into an Enum for image format number

An enum is always a better idea as it allows the compiler to do
some checks, and as this info is internal only to the WRLib it
will not change the API.

wrlib/imgformat.h

@@ -30,14 +30,16 @@
 #define IMGFORMAT_INTERNAL_H
 
 
-#define IM_ERROR   -1
-#define IM_UNKNOWN  0
-#define IM_XPM      1
-#define IM_TIFF     2
-#define IM_PNG      3
-#define IM_PPM      4
-#define IM_JPEG     5
-#define IM_GIF      6
+typedef enum {
+	IM_ERROR   = -1,
+	IM_UNKNOWN =  0,
+	IM_XPM     =  1,
+	IM_TIFF    =  2,
+	IM_PNG     =  3,
+	IM_PPM     =  4,
+	IM_JPEG    =  5,
+	IM_GIF     =  6
+} WRImgFormat;
 
 /* How many image types we have. */
 /* Increase this when adding new image types! */

wrlib/load.c

@@ -68,7 +68,7 @@ static int RImageCacheMaxImage = -1;	/* 0 = any size */
 static RCachedImage *RImageCache;
 
 
-static int identFile(char *path);
+static WRImgFormat identFile(char *path);
 
 
 char **RSupportedFileFormats(void)
@@ -272,7 +272,7 @@ char *RGetImageFileFormat(char *file)
 	}
 }
 
-static int identFile(char *path)
+static WRImgFormat identFile(char *path)
 {
 	FILE *file;
 	unsigned char buffer[32];