pHYs fix

New behaviour dictated: for files processed with Scale2x print resolution is doubled, with Scale3x - tripled, therefore print size remains the same. If pHYs chunk in original PNG is absent, 96 ppi is assumed as source resolution.

For Windows users, compiled Windows 64-bit exe builds are added. Windows 64-bit exe are quite self-contained and do not change a thing in your system, so simply start the installer and extract them anywhere you want, you can easily move or delete them as you wish later.