Renamed window context field from "gl" to the more generic "ctx"

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Cleaner context creation

 - No direct context creation call
 - Window calls apropriate internal function
 - Context has method to create share context

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>