Clear framebuffer after macro OSD disappears

[kbeckmann]

Some scaling modes don't redraw all pixels of the framebuffer.

If the macro osd has been rendered, there could be some areas that are not redrawn left visible on the screen.

There are a few approaches to solve this. We should probably clear the framebuffer after the OSD disappears, but make sure that it also gets drawn by the emulator so we don't get a blinking frame or so.