kmssink: Save last metadata at the same time as the last buffer

The render width/height and the vinfo was only saved upon renegotiation. This fixes the problem by saving this metadata at the same time the buffer is saved. The saved copy of this is needed for expose() and drain() virtual functions. This fixes various assertion that happens on drain query.

This is fixing the part I could reproduce in !1028 (closed), this fix is quite self explained, and clearly something I didn't do right in the recent rework to support dynamic cropping.