renderer: replace render buffer indices with queues
renderer: simplify code by always maintaining one front buffer
renderer: remove now pointless GetNext(Decode/Render) function
renderer: name present buffer structure to be able to reference it
renderer: store bypass format in rendermanager instead

The present method will be overwritten on each flippage
renderer: drop copies of variables, use queue data directly
renderer: grab presentlock in AddVideoPicture
[Linux] Read OS information from /etc/os-release
Most modern Linux distributions use the /etc/os-release
file for operating system information.
This patch will try to read the PRETTY_NAME variable from
/etc/os-release to identify the Linux distribution.

If /etc/os-release doesn't exist, the code will still fall back
to lsb_release or the distro-specific release files.

http://www.freedesktop.org/software/systemd/man/os-release.html
http://0pointer.de/blog/projects/os-release.html
Merge pull request #2900 from mus65/os-release

[Linux] Read OS information from /etc/os-release
Merge pull request #2811 from FernetMenta/erenderbuf

renderer: replace ring buffer logic with queues for free, queued, and discarded buffers
[Settings] Make Settings level lockable
[Settings] Treat 'Set as default' in CGUIDialogXXXSettings as expert level