plug-ins: set blue channel of BC5 dds images to 0 instead of 255.

Information in issue #6200 revealed that 0 as default value for
the blue channel is a lot more common than 255 so let's use 
that instead. The discussion and testing revealed no negative
effects for the other formats that use the same code to
initialize the memory to read blocks of image data.

plug-ins/file-dds/dxt.c

@@ -1484,7 +1484,7 @@ dxt_decompress (unsigned char *dst,
     {
       for (x = 0; x < width; x += 4)
         {
-          memset(block, 255, 16 * 4);
+          memset(block, 0, 16 * 4);
 
           if (format == DDS_COMPRESS_BC1)
             {