loadimage show pcx error

[a740g]

@macalwen This is fixed! I'll run some more tests and then push the fix to the repo, and it should be available in the next version we publish.

Basically, dr_pcx has a bunch of issues with PCX formats other than the standard ones. I initially, set out to fix these shortcomings by studying the loaders in Gimp, Paint.Net and some others. However, I ended up re-writing the loader.