hold/release callbacks in pl_frame

Open question:

• Do map/unmap also need to be able to create the pl_tex itself?
• Should we make an effort to try and early-release textures if no user hook depends on those planes?
• If there are user hooks that depend on video planes, do we emit a blit? Or do we delay releasing? (This might stall decoding if we hold on to the video plane for the entirety of some 16ms runtime meme shader)