Fix gamut masks in advanced color selector

Commit 8441c26f introduced a bug into gamut mask handling in the advanced color selector: the mouse position used to determine if a color is selectable is not correctly adjusted, that causes a mismatch between shapes shown on the wheel (which are correct) and selectable area.

Test Plan

1. open advanced color selector and gamut masks docker
2. select a gamut mask
3. in the advanced color selector, drag the selection circle to the edges of the mask shape

• => the circle must stay within the boundaries of the mask shape (+- few pixels)
• => the selector must allow to select all colors inside the mask shape

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.