Lightweight Java Game Library Code

Linux: Fix context NULL'ed before free'ing

Fix display mode switching and use separate X connection for display mode switching on linux

Linux part:

Merged Window and Display into org.lwjgl.Display
Added support for persistent context. This will enable LWJGL games
to avoid recreating the context resources like textures and display
lists when switching between windowed and fullscreen and/or context
dimensions.

Added PixelFormat class instead of individual parameters to Display.create
and Pbuffer constructor.