(core+platform) refactor gluse to video platform

First out of many refactor commits
where we try to break the openGL usage
away from being nesten in arcan_lua/arcan_frameserver/
arcan_3dbase/arcan_video into its own platform.

The reason for this is partly to more easily
support different underlying formats (e.g. mantle)
and to open up for newer GL version / context configuration
without breaking backwards compatibility.