Window jumps to (0,0) upon first animation

[dccsillag]

I've finally been able to reliably reproduce this.

Follow the steps:

• Create a window;
• Move it;

Instead of simply animating from where it was to where you moved it, the window will have jumped to (0,0) and then animated to where it was supposed to be.

[dccsillag]

It's worth noting that this does not happen in @alexfrederiksen's PR. After looking at his code, my guess is that it has something to do with the initialization of the w->animation_* members in win.c.

[dccsillag]

Solved.

What happened is that animation_{x,y} were both equal to 0. Now they get properly initialized via pending_g.
Also, we take some care regarding animation_progress -- values in [0,1) denote that we are in the middle of an animation, while anything else means that we are not. Plus some care in managing old_win_image.