When user resizes canvas to a bigger size than the current canvas with offset anchor at the center, the resulting document doesn't get bigger with equidistant units from center.

**To Reproduce**

1) Open Krita and a new document of 100 px X 100px
2) Fill canvas with black (just to help visually)
3) Open resize canvas dialog and put 200 px X 200 px in height width.
4) Click on the center anchor and click okay

**Expected result**

Resulting canvas should be resized equally from center

**Actual results**

canvas is resized more towards right, like shown below.

 ___________
|  _____    |
| |     |   |
| |     |   |
| |_____|   |
|           |
|___________|


I am using git master on Opensuse leap 42.3