Don't use refs for unmanaged bitmaps

[Nycto]

This reduces the number of small memory allocations that happens when working with bitmaps. Bitmaps are split into two categories: managed and unmanaged. Unmanaged bitmaps are passed around by copying pointers, managed bitmaps are passed around using nim object refs.

[ninovanhooff]

Hey thanks, seems this might improve both readability and performance somewhat.

I think I did find a pre-existing bug tho for a bitmap that should be managed

[Nycto]

@ninovanhooff I've updated to incorporate all your feedback. And you were right about the bitmask image. We may want to publish a new patch version that fixes that bug, as it's a fairly significant memory leak

[ninovanhooff]

@Nycto allright, would create a PR for the bitmask leak?

PS: I prefer no force-pushes, so that I can see what changed since the last review. If you want to keep a clean git history, perhaps squash merge?

[Nycto]

I’ll skip the force push in the future— sorry about that. This change includes the bitmap fix, so once I merge I’ll cut a new release