fill background of intermediary blitbuffers

we need to explicitly fill those buffers, now that white is not "0"
anymore.

frontend/ui/widget/textboxwidget.lua

@@ -209,6 +209,7 @@ function TextBoxWidget:_render(v_list)
     local space_w = RenderText:sizeUtf8Text(0, Screen:getWidth(), self.face, " ", true).x
     local h = (font_height + line_height_px) * #v_list
     self._bb = Blitbuffer.new(self.width, h)
+    self._bb:fill(Blitbuffer.COLOR_WHITE)
     local y = font_height
     local pen_x = 0
     for _,l in ipairs(v_list) do