Add Heatmap RenderPrimitive (chunk mtime)

[DragonDev1906]

As suggested in #1665 I've added a heatmap showing recent chunks.

• Based on the chunk mtime (instead of using the chunk inhabited time, which would have required calculating a min/max)
• Configured by setting an earliest timestamp (alpha=0) and a current timestamp (alpha=255)
• To make the implementation easier only the alpha value is changed with red as a base color (see Screenshot)
• You might have to use forcerender, otherwise the layer might not update correctly (maybe deleting the related files in the output might work, too)
• Could someone test this with a more active map? My testmap is rather small and had only 3 different chunk mtimes, which resulted in the sharp edges shown below. I would like to see how it looks on a more active map.

Example configuration:

renders["spawnable"] = {
   "world": "Test12985",
   "rendermode": [ClearBase(), HeatmapOverlay(
      # t_invisible=int((t_now - timedelta(days=30)).timestamp()),
      # t_full=int(t_now.timestamp()),
   )],
   "title": "Heatmap Overlay",
   "overlay": ["normalrender"],
}

[CounterPillow]

Sorry it took a while to review this. I haven't personally run it yet, but there's two things I've spotted which are minor issues before I did. Thanks for your PR!

[DragonDev1906]

I applied the suggested changes and rebased the PR to the current version (to test if it still works)

[CounterPillow]

Thanks. I'll try to render a map with it today to see it in action, and then merge it if it's all good

[CounterPillow]

overviewer_core/src/primitives/overlay-heatmap.c: In function ‘get_color’:
overviewer_core/src/primitives/overlay-heatmap.c:48:5: warning: implicit declaration of function ‘PY_XDECREF’; did you mean ‘Py_XDECREF’? [-Wimplicit-function-declaration]
     PY_XDECREF(mtime_pyobj);
     ^~~~~~~~~~
     Py_XDECREF

Small mistake, please change and git commit -a --amend, then git push -f to force-push your fixup.

Trying this out on a map now anyway with it fixed locally.

[DragonDev1906]

Thank you for pointing this out, apparently I forgot to run python setup.py build

[CounterPillow]

Seems to work fine, aside from the fact that Minecraft is so write-happy you rarely get smooth transitions since essentially every chunk a player can see will be written.

[DragonDev1906]

Minecraft probably has to save whenever there is any entity in the chunk (or might just save always to make programming easier).

[CounterPillow]

It famously updates the chunk whenever a cow moos, from what I've heard