Register fbdev allocator, and set hardware_overlay to false

Register the allocators used by fbdevframebuffersink. Set the
default value of the hardware_overlay property to false in
fbdev2sink (ideally the property could be removed entirely when
using fbdev2sink).
Don't wait for page-flip completion in drmsink, add benchmark mode

Don't wait for completion of page-flips in drmsink. At the moment,
when a show frame request happens while previous page-flip hasn't been
compeleted yet, the current frame is dropped.

Add benchmark property to test video/system memory read and write
speed. This helps to give more insight into performance and whether
a buffer pool in video memory is advantageous.