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/data-types.h

@@ -81,6 +81,9 @@ typedef struct ImageAnchorPosition {
 #define DECORATION_FG_CODE 58
 #define CHAR_IS_BLANK(ch) ((ch) == 32 || (ch) == 0)
 
+// PUA character used as an image placeholder.
+#define IMAGE_PLACEHOLDER_CHAR 0x10EEEE
+
 #define FG 1
 #define BG 2
 
@@ -192,6 +195,7 @@ typedef union LineAttrs {
     struct {
         uint8_t is_continued : 1;
         uint8_t has_dirty_text : 1;
+        uint8_t has_image_placeholders : 1;
         PromptKind prompt_kind : 2;
     };
     uint8_t val;