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

frontend/ui/widget/textwidget.lua

@@ -21,6 +21,7 @@ local TextWidget = Widget:new{
 --function TextWidget:_render()
     --local h = self.face.size * 1.3
     --self._bb = Blitbuffer.new(self._maxlength, h)
+    --self._bb:fill(Blitbuffer.COLOR_WHITE)
     --self._length = RenderText:renderUtf8Text(self._bb, 0, h*0.8, self.face, self.text, true, self.bold)
 --end