Image placement using Unicode placeholders

This commit introduces the Unicode placeholder image placement method.
In particular:
- Virtual placements can be created by passing `U=1` in a put command.
- Images with virtual placements can be displayed using the placeholder
  character `U+10EEEE` with diacritics indicating rows and columns.
- The image ID is indicated by the foreground color of the placeholder.
  Additionally, the most significant byte of the ID can be specified via
  the third diacritic.
- Underline color can be optionally used to specify the placement ID.
- A bug was fixed, which caused incomplete image removal when it was
  overwritten by another image with the same ID.

kitty/parse-graphics-command.h

@@ -39,7 +39,8 @@ static inline void parse_graphics_code(Screen *screen,
     cell_x_offset = 'X',
     cell_y_offset = 'Y',
     z_index = 'z',
-    cursor_movement = 'C'
+    cursor_movement = 'C',
+    unicode_placement = 'U'
   };
 
   enum KEYS key = 'a';
@@ -124,6 +125,9 @@ static inline void parse_graphics_code(Screen *screen,
       case cursor_movement:
         value_state = UINT;
         break;
+      case unicode_placement:
+        value_state = UINT;
+        break;
       default:
         REPORT_ERROR("Malformed GraphicsCommand control block, invalid key "
                      "character: 0x%x",
@@ -268,6 +272,7 @@ static inline void parse_graphics_code(Screen *screen,
         U(cell_x_offset);
         U(cell_y_offset);
         U(cursor_movement);
+        U(unicode_placement);
       default:
         break;
       }
@@ -327,7 +332,7 @@ static inline void parse_graphics_code(Screen *screen,
 
   REPORT_VA_COMMAND(
       "s {sc sc sc sc sI sI sI sI sI sI sI sI sI sI sI sI sI sI sI sI sI sI sI "
-      "si sI} y#",
+      "sI si sI} y#",
       "graphics_command", "action", g.action, "delete_action", g.delete_action,
       "transmission_type", g.transmission_type, "compressed", g.compressed,
       "format", (unsigned int)g.format, "more", (unsigned int)g.more, "id",
@@ -341,8 +346,9 @@ static inline void parse_graphics_code(Screen *screen,
       "num_cells", (unsigned int)g.num_cells, "num_lines",
       (unsigned int)g.num_lines, "cell_x_offset", (unsigned int)g.cell_x_offset,
       "cell_y_offset", (unsigned int)g.cell_y_offset, "cursor_movement",
-      (unsigned int)g.cursor_movement, "z_index", (int)g.z_index, "payload_sz",
-      g.payload_sz, payload, g.payload_sz);
+      (unsigned int)g.cursor_movement, "unicode_placement",
+      (unsigned int)g.unicode_placement, "z_index", (int)g.z_index,
+      "payload_sz", g.payload_sz, payload, g.payload_sz);
 
   screen_handle_graphics_command(screen, &g, payload);
 }