added menu option to take a screenshot

-android os built in screenshot wasnt always preserving all the gfx
-this just copies pixels on the next draw
-all the work is done on the render thread (otherwise gl context is lost)
-jpeg was faster than png
-doesnt dismiss popup incase user wants to take a few screenshots
-only saves the gles gfx (none of the views in front like vmu/fps/menu; does include osd buttons)