Added support for multires baking and CANCEL event.

I initially tried to fire events (ie call BLI_callback_exec) only from the RE_* functions, because that's how it is done for rendering-related events. And I found it cleaner since it would ensure that even if baking functions are called from somewhere else than the object bake operator, the events would still be fired. At least it ensures that they are fired both when executing and invoking the bake operators.

But a lot of the baking mechanism, especially for multires baking, is coded directly within the operator, so it became easier to put the calls to BLI_callback_exec. I also had to add a pointer to bmain, scene and stop in the MultiresBakeJob struct. Events are fired from (multires)bake_startjob and (multires)bake_freejob, as you advised, as well as from multiresbake_image_exec_locked and bake_image_exec for when the operator is executed (startjob/freejob are only called in the case of invoked operator).

Now, you right: this is for blender internal only. I just understood that cycles baking is handled by the object_bake_api.c file, and not object_bake.c where I've been working so far. I'm going to address this in my next update.

And here is the support for cycles renders! Much more straight forward once I got how all of this is organized. :)

So, at this point the main thing I am not so confident in is the way I detect whether the job has been canceled or not. Is seems to be hacky sometimes, and a different hack for all cases.

PS: As for felixg.david, I believe he intends to shift to the next frame and call the bake() op every time the previous bake completed