patch 9.0.0139: truncating virtual text after a line not implemented

Problem:    Truncating virtual text after a line not implemented.
            Cursor positioning wrong with Newline in the text.
Solution:   Implement truncating.  Disallow control characters in the text.
            (closes #10842)
diff --git a/src/structs.h b/src/structs.h
index 71114b0..e6b9f0d 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -815,6 +815,8 @@
 
 #define TP_FLAG_WRAP		0x40	// virtual text wraps - when missing
 					// text is truncated
+#define PROP_TEXT_MIN_CELLS	4	// minimun number of cells to use for
+					// the text, even when truncating
 
 /*
  * Structure defining a property type.