Calculate first clickable point, from the top-left

Summary:
Wine is using XWindow Shape Extension for transparent tray icons.
We need to find first clickable point starting from top-left.

BUG: 399234

Test Plan:
Bug fix test:
1. Enable KWin compositing;
2. Download this test program (SHA-1: 63ae606aee64259091e7f82436d4ecdf3a6e9047): https://www.nirsoft.net/utils/tflash210.zip
3. Run the .exe with Wine **Staging**
4. Right click on the icon
5. Context menu should show

Regression test:
1. Run XChat
2. Right click
3. Context menu should show in the same place as without patch

Reviewers: davidedmundson, #plasma_workspaces

Reviewed By: davidedmundson, #plasma_workspaces

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22767