patch 8.1.2039: character from 'showbreak' does not use 'wincolor'

Problem:    Character from 'showbreak' does not use 'wincolor'. (Nick Jensen)
Solution:   Mix with 'wincolor'. (closes #4938)
diff --git a/src/screen.c b/src/screen.c
index 14b9465..22b26b4 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4153,6 +4153,9 @@
 		     * required when 'linebreak' is also set. */
 		    if (tocol == vcol)
 			tocol += n_extra;
+		    // combine 'showbreak' with 'wincolor'
+		    if (win_attr != 0)
+			char_attr = hl_combine_attr(win_attr, char_attr);
 #  ifdef FEAT_SYN_HL
 		    // combine 'showbreak' with 'cursorline'
 		    if (cul_attr != 0)