Fixed crashes with malformed MCA files.

Some tools write MCA files that are not 4K-aligned, these caused a crash.
added default suffix for world save to PNG

fix #34: now a default suffix ".png" is added when a filename without a
suffix was given.
Merge pull request #36 from EtlamGit/SaveAddMissingPNG

added default suffix for world save to PNG
Merge pull request #33 from EtlamGit/master

New renderer implementation for Mob Spawning
Merge pull request #31 from madmaxoft/master

Fixed crashes with malformed MCA files.