Support remote image by OSC 5379 show_picture sequence.

fm.h

@@ -373,6 +373,8 @@ typedef struct _imageCache {
     int index;
     short width;
     short height;
+    short a_width;
+    short a_height;
 } ImageCache;
 
 typedef struct _image {
@@ -919,6 +921,7 @@ global char *CurrentKeyData;
 global char *CurrentCmdData;
 global char *w3m_reqlog;
 extern char *w3m_version;
+extern int support_remote_image;
 
 #define DUMP_BUFFER   0x01
 #define DUMP_HEAD     0x02