patch 8.0.1777: cannot cleanup before loading another colorscheme

Problem:    Cannot cleanup before loading another colorscheme.
Solution:   Add the ColorSchemePre autocommand event.
diff --git a/src/syntax.c b/src/syntax.c
index e945dac..bb695b5 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -7224,6 +7224,8 @@
     buf = alloc((unsigned)(STRLEN(name) + 12));
     if (buf != NULL)
     {
+	apply_autocmds(EVENT_COLORSCHEMEPRE, name,
+					       curbuf->b_fname, FALSE, curbuf);
 	sprintf((char *)buf, "colors/%s.vim", name);
 	retval = source_runtime(buf, DIP_START + DIP_OPT);
 	vim_free(buf);