[rbp] Check free memory, and if plenty use full size buffers for reading files

omxplayer has used a quarter sized video buffer and a half sized audio buffer for the file reading message queue, compared to dvdplayer.
This is required on 256M boards, but is unneccessary on 512M board. So we query the free memory and use full size buffers if there is plenty.
Revert "[rbp] Always use the aspect ratio from file."

This reverts commit 7f5d3ede514f30552e42464c29227d570b38ed8a.
[wip] hacking around