[ColorPicker] click on strict corner is difficult

[DefeninMaxime]

Describe the bug
It's very difficult to get a color in corner of picker. For example rgb(255,255,255) at top left corner.

To Reproduce
Steps to reproduce the behavior:

1. Open color picker
2. Click on the corner left top (for example)

Expected behavior
I could choose a color in corner.

[Markus Mohoritsch]

Thats still true in current master. The Problem seems to be that it's impossible to sample pure white or black from the canvas.

I will try to fix this.