Bug 459135

SUMMARY
Setting the cache size to a stupidly low setting (e.g. 10px) will cause Krita to crash when generating cache.

STEPS TO REPRODUCE
1. Open animated KRA file.
2. Set `limit cache frame size` to a really low value, like 10 px
3. Press the play button.

OBSERVED RESULT
It will hit an assert with the following line:

```
ASSERT (krita): "extraWeight == 1" in file /home/appimage/workspace/Krita_Release_Appimage_Build/krita/libs/image/kis_paint_device.cc, line 763
```

EXPECTED RESULT

At the very least, it won't crash.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Manjaro KDE
Running from stable appimage. 

ADDITIONAL INFORMATION

Some questions to keep in mind regarding how we should be handling this scenario.

Q1: Stupidly low settings like 10 pixels are basically unusable, so we should reconsider what the lowest value someone should use here. Notably, it wouldn't be very nice looking to be using a 10px frame cache size, so is it really worth it? Personally, I find 128px to be the lowest possible texture size to be still usable.

Q2: Stupidly low settings could be resolved by changing our mindset and instead use a percentage of the user's display as a basis for what size to render. Ultimately, this would make this setting easier to use as you could easily reuse settings between different machines with very different resolutions. With this in mind, probably something like 5% of the users current display resolution should be a reasonable value (10% perhaps, which would be a resolution of 108px on a 1080p display.)