[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
Merge pull request #5093 from popcornmix/double_buffer

[rbp] Reduce GPU memory use when limited