Incorrect brush sizes relative to zoom level

[sxxov]

When "Size relative to canvas" is checked, zooming in/out will in/decrement the brush size by an incorrect amount — 9 instead of 10. This causes a mismatch of zoom level to brush size when zooming out & back in.

This affects 1.9.0 & 1.9.1.

[voineaandrei]

In addition to this, while zooming in/out repeatedly at low zoom levels will make the brush size get huge values and eventually cause an integer overflow. In this state, the brush size will stay blocked at 1. The only way to unblock it is to select a value on the slider.