Creating an FBO may be expensive, so we should avoid the operation
if possible. When transitioning between theme nodes, the widget's
opacity is used to paint to the offscreen textures which are blended
together - this means that the textures have to be recreated each time
the widget's opacity changes. It is much more effective to paint the
textures at full opacity and respect the widget's paint opacity when
blending the textures together.
https://bugzilla.gnome.org/show_bug.cgi?id=627085
The framebuffer setup code can fail, e.g. when the texture used as
color buffer has a height/width of 0. In that case, the call to
cogl_pop_framebuffer() will crash the shell trigger an assert. Add
error checking to fail gracefully.
Our behavior of reversing the animation when the widget's style
changes back to the previous one is sound. On the other hand, when
there's a change to a new style while a transition is active, we
simply cancel the ongoing transition. Updating the transition
correctly so that the new one starts from an intermediate state
is hard.
Nevertheless, if the style changes before any time of the transition
has elapsed, we should do better than the current behavior.
https://bugzilla.gnome.org/show_bug.cgi?id=621140
