ksw/convert fitting Gaussian fwhm to semi-axes to render ellipse region

[kswang1029]

Description
This is to fix #2212. The root cause is the ellipse region takes semi-axes length to define the size. So we need to convert the fitted Gaussian FHWM by multiplying 0.5.

Checklist

For linked issues (if there are):

• assignee and label added
• ZenHub issue connection, board status, and estimate updated

For the pull request:

• reviewers and assignee added
• ZenHub estimate, milestone, and release (if needed) added
• e2e test passing / corresponding fix added
• changelog updated / no changelog update needed
• protobuf updated to the latest dev commit / no protobuf update needed
• BackendService unchanged / BackendService changed and corresponding ICD test fix added

[YuHsuan-Hwang]

Yes, should be half of the size. Thanks for fixing this!