Start work on specifying a color for extra cursors

kitty/screen.h

@@ -92,9 +92,15 @@ typedef struct ExtraCursor {
     index_type cell;
 } ExtraCursor;
 
+typedef struct ExtraCursorColor {
+    color_type color;
+    bool is_set, is_none, is_lookup;
+} ExtraCursorColor;
+
 typedef struct ExtraCursors {
     ExtraCursor *locations;
     unsigned count, capacity;
+    struct { ExtraCursorColor cursor, text; } color;
     bool dirty;
 } ExtraCursors;