gl: Use GL_TEXTURE_2D instead of GL_TEXTURE_RECTANGLE

We create our textures (in Desktop GL) with GL_TEXTURE_RECTANGLE,
vaapi attempts to bind our texture to GL_TEXTURE_2D which throws a
GL_INVALID_OPERATION error and as thus, no video.

Also, by moving exclusively to GL_TEXTURE_2D and the npot extension
we also remove a difference between the Desktop GL and GLES2 code.

https://bugzilla.gnome.org/show_bug.cgi?id=712287
cocoa: allow to call set_window_handle before to create glcontext

So that it actually make cocoa videooverlay example work again.

Fixes bug https://bugzilla.gnome.org/show_bug.cgi?id=719758
glvideomixer: Use correct coordinates after the switch from GL_TEXTURE_RECTANGLE to GL_TEXTURE_2D

https://bugzilla.gnome.org/show_bug.cgi?id=720177
osx: allow x11/glx backend and set default runtime to cocoa/nsgl

https://bugzilla.gnome.org/show_bug.cgi?id=719757