patch 9.0.0247: cannot add padding to virtual text without highlight

Problem:    Cannot add padding to virtual text without highlight.
Solution:   Add the "text_padding_left" argument. (issue #10906)

src/structs.h

@@ -800,7 +800,8 @@ typedef struct memline
 typedef struct textprop_S
 {
     colnr_T	tp_col;		// start column (one based, in bytes)
-    colnr_T	tp_len;		// length in bytes
+    colnr_T	tp_len;		// length in bytes, when tp_id is negative used
+				// for left padding plus one
     int		tp_id;		// identifier
     int		tp_type;	// property type
     int		tp_flags;	// TP_FLAG_ values