gles: Avoid crash when capturing snapshot when using dvdplayer

Note: snapshot will be blank, but that's better than crashing
[omxcodec] Reduce GPU memory use by 2 video frames
[rbp] Reduce GPU memory use when limited

Switching from default triple buffered output to double buffered saves 8M with 1080p GUI.
This may slightly reduce framerate, but is likely to be minimal.

Assume if gpu_mem is set below the default 128M that this memory reduction is wanted