Make the TextCache object available in the Screen, LineBuf and Line objects

kitty/screen.h

@@ -105,6 +105,7 @@ typedef struct {
     Cursor *cursor;
     Savepoint main_savepoint, alt_savepoint;
     PyObject *callbacks, *test_child;
+    TextCache *text_cache;
     LineBuf *linebuf, *main_linebuf, *alt_linebuf;
     GraphicsManager *grman, *main_grman, *alt_grman;
     HistoryBuf *historybuf;