Calculate rotation center if it's not supplied

[rschamp]

When a new skin is added, previously the rotation center was set to [0, 0]. In the case of costumes and backdrops added from libraries, the center should be calculated from the bounding box of the imported skin.

Toward scratchfoundation/scratch-gui#18

[cwillisf]

I think this is fine as is, but I wonder if it would be better to always calculate the rotation center as [w/2, h/2] initially rather than take a flag for that. If the client immediately overrides that then that's fine - it wasn't a very expensive computation to begin with.

One complication would be if the client creates a skin, then sets the rotation center, then the skin finishes downloading: we'd need to avoid overriding the user-set rotation center in that case. Maybe that added complexity isn't worth it.

Thoughts?

[rschamp]

One complication would be if the client creates a skin, then sets the rotation center, then the skin finishes downloading: we'd need to avoid overriding the user-set rotation center in that case.

I thought about always calculating the center, but it would require us to deal with the edge case of manually setting the center while the skin is still downloading (since we would start the download and then immediately set the rotation center here).

What if we passed in rotationCenter to createSkinFromURL rather than the calculateRotationCenter flag, and calculate it if rotationCenter is undefined? It wouldn't help the case of someone changing the center before the skin downloads, but in this situation it would be an edge case we could deal with later.

[cwillisf]

What if we passed in rotationCenter to createSkinFromURL rather than the calculateRotationCenter flag, and calculate it if rotationCenter is undefined?

That sounds good to me.