feat: add frame_converter and provide consumers opaque frame type

[Julusian]

This is the first polished portion of https://github.com/CasparCG/server/pull/1581

This removes the image_data on const_frame and replaces it with an opaque image_ptr. Consumers can use the frame_converter to convert that into a pixel buffer. For now this is the same basic texture to buffer copy as before, #1610 will replace that with a proper conversion step

Most of the consumers need testing to make sure they work and don't crash immediately. ndi and ffmpeg appear to not work for me, but the same is true with master so is probably something weird on my machine today..