ignore ':' in col 1 of vips7 paths

oh argh, we were mangling windows paths

see https://github.com/jcupitt/build-win32/issues/11

libvips/deprecated/vips7compat.c

@@ -69,6 +69,8 @@
  * 	  -> c:\silly ""
  * 	c:\silly
  * 	  -> c:\silly ""
+ * 	C:\fixtures\2569067123_aca715a2ee_o.jpg
+ * 	  -> C:\fixtures\2569067123_aca715a2ee_o.jpg ""
  *
  * vips8 handles this in a much better way :( 
  */
@@ -109,7 +111,11 @@ im_filename_split( const char *path, char *name, char *mode )
 				break;
 		}
 
-	if( *p == ':' ) {
+	/* Ignore a ':' in column 1, it's probably a drive letter on a
+	 * Windows path. 
+	 */
+	if( *p == ':' &&
+		p - name != 1 ) {
                 vips_strncpy( mode, p + 1, FILENAME_MAX );
                 *p = '\0';
         }

test/test_iofuncs.py

@@ -51,7 +51,10 @@ class TestIofuncs(unittest.TestCase):
                  ""]],
             ["c:\\program files\\x:hello",
                 ["c:\\program files\\x",
-                 "hello"]]
+                 "hello"]],
+            ["C:\\fixtures\\2569067123_aca715a2ee_o.jpg",
+                ["C:\\fixtures\\2569067123_aca715a2ee_o.jpg",
+                 ""]]
         ]
             
         for case in cases: