Support big images

[AsumFace]

I've run into some integer overflow issues with color.d and png.d when trying to work with images that have slightly bigger dimensions than normal, this PR fixes those issues.

• setPixel doesn't produce bogus array indices for sane input anymore.
• saving and loading big images works now (on 64 bit platforms). In a basic test using a 30000x20000 RGBA image, the image could be saved correctly and decoded correctly both in it's original encoding and in reencoded (by external tool) form. Before the fix the output file was terminated abruptly and broken.

[adamdruppe]

Thanks! I am pretty sloppy about the size_t/int thing.