CoglTexturePixmapX11: add support for stereo content

Add cogl_texture_pixmap_x11_new_left() and
cogl_texture_pixmap_x11_new_right() (which takes the left texture
as an argument) for texture pixmap rendering with stereo content.
The underlying GLXPixmap is created using a stereo visual and shared
between the left and right textures.
examples/cogl-x11-tfp: Add a --gears option to run glxgears

If --gears is passed on the command line, glxgears is run, and
is used for the source window rather than drawing solid rectangles
into a window we create outselves.
examples/cogl-texture-pixmap-x11: Add a --stereo command line option

If --stereo is passed, then the texture pixmap is created as a stereo
texture pixmap, and also, if passed in conjunction with --gears,
glxgears is also run with the -stereo option.