Implement libpng

[blast007]

We currently have our own code for both encoding (when creating screenshots) and decoding (for loading textures) PNG files. We should make use of libpng instead.

• [x] Read textures with libpng
• [x] Write screenshots with libpng
• [x] Clean up existing textures that generate warnings
• [x] Hook into libpng's error/warning reporting to show errors/warnings in the game client

[blast007]

PR #84 implemented libpng for reading textures and writing screenshots. Writing screenshots is roughly 2 to 3 times faster now, while still resulting in a similar file size.

From implementing libpng, I also became aware that some of our PNG files have some minor issues. Several libpng warnings are currently printed to stderr from just the textures included with the game. I am considering if we should hook into libpng's error/warning handling to show errors and warnings in the game client console (possibly in addition to stderr).