Crashes on high sixel output

[ghost]

On the 0.0.10 release binary, cat'ing https://jexer.sourceforge.io/sixel/sixel_6x13.cap caused darktile to crash. There are a few sixel tests on that page at different resolutions, and also some sixels with transparency if you would like to test that.

I also saw a pegged CPU and unresponsive output when trying an animation. I loaded up flame.gif, it ran for a bit, but then CPU started spiking and I eventually had to kill it from another terminal. Here is what it was supposed to behave like.