Fix display scale updates in eglvivsink

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

glUniform2f() was being called in the wrong place, where no valid
GLES context was currently set. Moving this to the frame render
function fixes this problem, since a valid context exists there.

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>