qml: activate item layer when transitions are running in MainDisplay and Player

Currently, transitions do not get realized as intended because in normal cases opacity setting is applied to child items. This, for example, causes images to reveal background shadow blur.

If we want to make a transition which adjusts opacity, the item should act as if it is a texture, so that this behavior does not happen.

This may also be an advantage, if the GPU is good at blending and sampling texture. The disadvantage is, if the layer texture gets updated, this will always incur an extra cost, because then it is not possible to get the benefits of caching (if the texture is not updated, the nodes do not need to get rendered at every opacity change).

I did not apply this globally in StackViewExt, because there is a problem with the software renderer that sometimes when layer is enabled, blending with background items does not work.