Fix showcase target overlay at wrong position

[luccasclezar]

Description

Doing some debugging I saw that the target overlay was being laid out starting from the center of the target. I fixed it simply by subtracting half the size from the offset.

I kept the FractionalTranslation there anyway because I didn't want to break anything, but now it only translates when size is null.

Fixes #346 Dont respond to any clicks in target

[vatsaltanna]

Hi @luccasclezar, The issue that you have mentioned in the PR doesn't have the issue with positioning, it's an issue of IgnorePointer widget which has been fixed in PR #351 and it will be available in the new release version so closing this PR for now Thanks for your contribution and support in the issue we really appreciate this.