Make canvasSize work on windows.

[dchapes]

Resolves #15

[dchapes]

Note: I use mercurial with hggit so I'm not sure if the file copies in the first commit correctly got detected as repository file copies on the conversion to git (so that things like git blame within the new files will know what lines haven't been changed since the original).

[dchapes]

Note the comment on #22 (comment) , these two PRs are slightly incompatible; I'll fix up one if you accept the other.

[dchapes]

I just noticed PR #3.
Sadly, although I tested this on a windows guest within VirtualBox it did have some issues with colours; perhaps the same issue as with the previous PR.
If it's just a matter of only having 16 colours I do have some code elsewhere that can shrink a palette. Perhaps further downgrading incoming images to 16 colours would be sufficient? If so I could try adding that, if not feel free to close this PR too.

Sorry for not looking at the previously closed PRs before submitting this one.