vulkan: encoder: misc clean ups mainly for GstVulkanEncodePicture
- vkencoder-private: add picture out buffer offset
- vkencoder-private: remove packed headers
- vkencoder-private: remove width, height and fps from GstVulkanEncodePicture
- vkencoder-private: remove nb_refs from GstVulkanEncodePicture
- vkencoder-private: remove pic_num and pic_order_cnt from GstVulkanEncodePicture
- vkencoder-private: remove is_ref member from GstVulkanEncoderPicture
- vkencoder-private: don't store output data size
- vkencoder-private: fix parameters overrides