sdl2: change_fonto_utf8(... FONTO_DEFAULT) -> zero size font
change_fonto_utf8() does not handle "default" font size, but uses flag-value 0 like it was real font size.
Patch meant also for S2_6 (I think this was a regression in 2.6.9 relative to 2.6.8)
change_fonto_utf8() does not handle "default" font size, but uses flag-value 0 like it was real font size.