fixes #619: correct size of trimmed sprites

Trimmed sprites had wrong size.
For example I had 100x100 png image which was trimmed to 50x50 (25 transparent pixels from each side).
The size of this sprite was 50x50.
This PR changes size (width and height) of trimmed sprite to size from before trimming (100x100 in this case).
It was also connected to #619.