Openlayers: Tiles flicker on load when semi-transparency is used

[rutgr]

Issue
When switching from v4.3.2 to v4.6.4 we notice a flicker has been introduced when using semi-transparent layers.
When zooming and panning over non-cached tiles you can clearly notice the opacity is applied after the tile has been loaded, resulting in a flickering effect.

Tested on Windows 7 with latest Chrome, FF, IE & Opera

Example
It is visible in OL's own examples: https://openlayers.org/en/latest/examples/semi-transparent-layer.html Try to zoom and pan over the semi transparent area to notice the issue.

[fredj]

To deactivate this transition effect, set the transition property to 0.
See 8fc664175001bba015b33d855c0f06feab1cd426