libweston: assert frame times never go backwards

Adding this check was prompted by
!609



There is no reason to allow frame times jump backwards, and apparently
we already have code that makes that assumption.

DRM KMS uses CLOCK_MONOTONIC as the vblank and page flip timestamps,
which by definition cannot go backwards. Other backends call
weston_compositor_set_presentation_clock_software().

Frame times are also reported directly to Wayland clients via
presentation-time extension, and clients too will not expect that the
timestamp could go backwards.

So make sure time can never go backwards.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>