tethering: add a function to get framebuffer in maru_sdl.

tethering: add a function to get framebuffer in maru_sdl.

When request_screenshot function is called by tethering module
it makes qemu update display. So, this function has to be implemented
to extract framebuffer because of unnecessary display update.

Change-Id: I642119a430261f0176be47445b3b6fe5630920cc
Signed-off-by: Kitae Kim <kt920.kim@samsung.com>