patch 8.1.0675: text property column in screen columns is not practical

Problem:    Text property column is screen columns is not practical.
Solution:   Use byte values for the column.
diff --git a/src/structs.h b/src/structs.h
index 4ac98b1..5b8c80b 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -705,7 +705,7 @@
  */
 typedef struct textprop_S
 {
-    colnr_T	tp_col;		// start column (one based)
+    colnr_T	tp_col;		// start column (one based, in bytes)
     colnr_T	tp_len;		// length in bytes
     int		tp_id;		// identifier
     int		tp_type;	// property type