Add warnings about Host Memory performance

[husberg-disguise]

All processing is done GPU-side, so supplying the API with host/CPU-side memory involves a lot of GPU upload/download traffic.

As of r19.1 Uncompressed transport types do not support host memory either. Edit: r19.2 will support host memory for Uncompressed

Add warnings to the API and documentation about this so integrations do not go through the trouble of downloading a GPU texture to host memory and then supplying it to the API which re-uploads it.

Host memory should still be supported for content that is generated purely CPU-side (unless removed completely).