uterm: video: add support for immediate buffer-swaps

uterm: video: add support for immediate buffer-swaps

Internally, we already support immediate buffer-swaps but this hasn't been
exported in the API. This patch adds an "immediate" argument that causes
the flip to be immediate and not synchronized with vertical-blanks.

Please note that this might block if there is a pending page-flip.
However, this is mostly a delay of 16ms so we can ignore it as this is
acceptable if you want immediate swaps while there is still a
pending-buffer.
You can listen for the UTERM_PAGE_FLIP event if you don't want this
behavior.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>