PNG image transparency lost during crop/fill operations

PNG image transparency lost during crop/fill operations resulting transparent color conversion to black color. Patches attached.

Here are some details. When cropped size is larger, then given image, then image is not enlarger (to preserve it's quality) and larger canvas image is created to place source image on it. Source image was containing alpha-channel information, but created canvas wasn't. When merged together alpha-channel information was totally lost and what was transparent in source image became black in target cropped image.