Fix display scale updates in eglvivsink

This is necessary for when the resolution changes during playback
(not just the aspect ratio)

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>
Drain VPU decoder frames when a new format is set

This is required for seamless playback; otherwise, decoded but not yet
displayed frames get lost when switching format (for example, during a
resolution change)

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>