patch 8.1.1426: no test for syntax highlight in popup window

Problem:    No test for syntax highlight in popup window.
Solution:   Add a screenshot test.  Update associated documentation. Avoid
            'buftype' being reset by setbufvar().
diff --git a/src/version.c b/src/version.c
index fa42ac2..0420cb6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -768,6 +768,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1426,
+/**/
     1425,
 /**/
     1424,