examples/cogl-x11-tfp: Fix compilation

Fix a couple of minor compilation problems from a bad rebase - a
missing label and a missing variable.
Add support for setting up stereo CoglOnscreens

If we want to show quad-buffer stereo with Cogl, we need to pick an
appropriate fbconfig for creating the CoglOnscreen objects. Add
cogl_onscreen_template_set_stereo_enabled() to indicate whether
stereo support is needed.

Add cogl_framebuffer_get_stereo_mode() to see if a framebuffer was
created with stereo support.

Add cogl_framebuffer_get_stereo_mode() to pick whether to draw to
the left, right, or both buffers.
Add a simple example of stereo drawing

Add a new example that draws a triangle floating in the air in
front of the screen.
CoglTexturePixmapX11: add support for stereo content

Add cogl_texture_pixmap_x11_new_stereo() to create a a texture
pixmap using a stereo visual, which can be used to render pixmaps for
windows with stereo content. The actual rendering is done by using
cogl_texture_pixmap_x11_create_right_texture(), which returns a
separate texture that renders with the right image.
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.