environment script: get back to two directories up after devtools.

As we go to $m/validate in that case.
keyframes: If the length of the line is inferior to one pixel,

don't draw it.
effects: Commit the timeline when adding an effect.
render: Add a static _factory_formats static field.

This field is used because of introspection problems in
fixating the static_pad_templates, for some reason we can't do
it more than once, so we keep the result stored.
bin: Do not build GI and PyGobject if not necessary
bin: Better handling of gst version and add default scenario path

We can now override needed version through en variables