Fix width/height for trimmed images

The width and height reported by `identify` for trimmed PNG images is incorrect because the transparent border is included in the new page size and offset. For example, for a 32x32 trimmed image, `identify` reports a geometry of 34x34+1+1. Also, because PNG images support canvas metadata but other formats may not (JPEG), the trimmed file is always explicitly converted to PNG so that the new calculation always works correctly.
Merge pull request #15 from jotson/master

Fix width/height for trimmed images