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/line-buf.c

@@ -52,6 +52,11 @@ linebuf_mark_line_clean(LineBuf *self, index_type y) {
     self->line_attrs[y].has_dirty_text = false;
 }
 
+void
+linebuf_set_line_has_image_placeholders(LineBuf *self, index_type y, bool val) {
+    self->line_attrs[y].has_image_placeholders = val;
+}
+
 void
 linebuf_clear_attrs_and_dirty(LineBuf *self, index_type y) {
     self->line_attrs[y].val = 0;