Introduce counter for the number of GstBuffers with VPU framebuffers inside

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>
Fix seeking and flushing ; VPU_DecFlushAll() works properly now

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

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