fix: the ItemViewport's corners is not smooth

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: lbwtw, zccrs

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files: - **[debian/deepin/OWNERS](https://github.com/linuxdeepin/dtkdeclarative/blob/master/debian/deepin/OWNERS)** Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[deepin-ci-robot]

deepin pr auto review

关键摘要:

• 在frag代码中使用 == 0.0 进行比较可能会导致一些逻辑问题，因为浮点数比较可能会因为精度问题而导致不准确的结果。建议使用 < 1.0 来替代。

是否建议立即修改:

• 是，建议立即修改以避免潜在的逻辑错误。