[Graphics] Selection outline color differs between linear and gamma colour space for the same values.

Reproduction steps:
1. Select a gameobject in the scene view.
2. Change colour mode (linear/gamma).
3. Notice that the selection outline colour changed.

Expected result: colour doesn't change.
Actual result: colour changes.

Reproduced with: 2018.1.0b13
Regression in 2018.1.0b13