add TexPlay#compare method to check if two images are the same (pixel-wise compare)

[banister]

image1.compare image2 #=> true (if same pixels)

[bil-bas]

This is something I actually already do somewhere, so having this in C would possibly be a boon. Not critical, though, since it is already available as image1.to_blob == image2.to_blob

[bil-bas]

Thinking about it, why not just overload ==?