[Bugfix] FS#13912 - splash_scroller with cyrillic crashes

I think they mean corrupted not crashes as I couldn't repro that

splash_scroller stepped by individual chars and split multibyte
sequences leaving corrupted chars

now it uses the new font_measurestring() to get the max_w and chops
it at break chars instead

this required a bit of a logic change to the function but should be
as good with less complications

Change-Id: I692db8b4737f41aec08dbc122fb844028498326d

apps/plugin.h

@@ -1040,6 +1040,8 @@ struct plugin_api {
 
     /* new stuff at the end, sort into place next time
        the API gets incompatible */
+    int (*font_measurestring)(const unsigned char *str, size_t maxbytes,
+                              size_t maxwidth, int *w, int *h, int fontnum);
 };
 
 /* plugin header */