videopool: add optional flags for surface pool allocation.

videopool: add optional flags for surface pool allocation.

Reword surface pool allocation helpers so that to allow for a simple
form, e.g. gst_vaapi_surface_pool_new(format, width, height); and a
somewhat more elaborated/flexible form with optional allocation flags
and precise GstVideoInfo specification.

This is an API/ABI change, and SONAME version needs to be bumped.