[OpenTK] Only init X11 if SDL2 is not used
[GL] Added GetPName.Max*ImageUniforms

These tokens were introduced in OpenGL 4.2 and 4.3.
[X11] Fixed SGI_swap_control query

GLX can return valid function pointer to unsupported functions.
We need to check for extension support in addition to the
function pointer.

Fixes crash on OpenSUSE 13.1 in VirtualBox.
Merge branch 'cursor' of https://github.com/thefiddler/opentk into thefiddler-cursor
[Win] Fixed MouseCursor.Default

It is now possible to switch from a custom cursor back to
MouseCursor.Default.