codec: vaapi: handle non preallocated pools

refs #28817 (closed) #28822 (closed)

On newer ffmpeg, the initial_pool_size might not be set when querying AVHWFramesContext.

Quoting avcodec_get_hw_frames_parameters() "If the hwaccel does not require pre-allocation, the field is left to 0, and the decoder will allocate new surfaces on demand during decoding."