Texplay does not use Gosu's improved interface

[jlnr]

Gosu has Image#to_blob and Image#insert(img, x, y) now. This makes it possible for Texplay to work with large images (>1024px) and could reduce coupling between Texplay and Gosu's internal implementation.

Just leaving this note here, because if anyone else is about to fix it, please sent me a note first. I could change the to_blob format last minute or fiddle with other optimizations to speed things up.