Changed screenshot format from BMP to PNG (using LodePNG)

NOTE: The little popup window ("Screenshot saved to disk as") still
displays a BMP filename, but it saves a PNG file to disk.
Merge branch 'screenshot-png' of https://github.com/atmaxinger/OpenRCT2 into atmaxinger-screenshot-png
allow BMP to still be saved with PNG as default
Merge branch 'atmaxinger-screenshot-png'