libgui: Added GLInfo, moved extensions and limits from client

Moved/improved old code from sys_opengl for checking OpenGL
extensions.

Extensions are now referred to by their official names, there is no
benefit in using shorter/nicknames.
libdeng2: Added ceilPow2
Refactor|GL|Client: Use libgui’s GLInfo instead of sys_opengl

Available extensions and OpenGL limits come now from libgui.

Also cleaned up old/dead code from sys_opengl.
libgui|Canvas: Initialize GLInfo