maxflow: add methods to get progression feedback while computing.

[Jehan]

Hi!

We are using libmaxflow in GIMP for a new tool (still marked experimental) for quick selection of objects. Unfortunately the tool is still somewhat slow, especially if we start working on bigger files and if not in "Performance" power mode for instance, or simply with older hardware. I had experience of waiting for 10 to 15 seconds and 90% of the waiting happens inside g->maxflow().

Now we'd like to work on performance and maybe I'll propose some patch. But in the meantime, we also have to accept that the bigger the image and slower the hardware, the higher are chances of slow rendering and it is acceptable. The problem still is that when you wait 10 or 20 seconds, if you don't know where you are in the processing, it feels very slow, and worse, the GUI won't be responding anymore, which will appear "frozen" and often desktops would show the infamous popup "GNU Image Manipulation Program is not responding" with the option to kill the process. Bottom line: GIMP feels broken.

The solution to this is to show progression information, which first shows where the rendering is at, and the GUI is not frozen anymore, no popup appears, etc. In fact even 20 seconds of rendering doesn't feel too long (when someone works on very big images, they are aware things may take longer and as long as they get progression feedback, they are not worried of being in some infinite loop bug!).

The submitted API does not change any existing API, but suggests an alternative to calling g->maxflow() which does the processing in one block (and we just have to wait without info). Instead, we can process like this now:

float progress;

graph->maxflow_prepare();
while (graph->maxflow_compute (&progress))
  gegl_operation_progress (operation, progress, (gchar *) "");

Basically maxflow_compute does the same as maxflow() except it returns regularly (but not too often either) with some progression value (between 0.0 and 1.0), and the calling program can do something with it (updating a GUI progression widget like in GIMP, or showing progression information in a CLI, etc.).

Here is what the tool looks like when using this new API in GIMP: https://peer.tube/w/nqPXSWgD7PxM98R3osZy99

The thing to notice is the small green circle progression info saying "Selecting" when the tool works. This is only possible with this new API. Also note that it's quite fast on this demo image, but when I worked with bigger images, I really had cases where it took over 10 seconds inside maxflow() and where the GUI was frozen, ending up with the "GIMP is not responding" popup. When you get this, the tool feels dreary.