Show tabs using ShaderEffectSource

This MR reverts back to ShaderEffectSource for showing webpage overviews in Tabs. This is due to the issues with images on HiDPI devices and as it was not always working. In particular, on HiDPI device the images are taken with low resolution and look pixelated in the overview. In addition, there are issues with pages not being fully loaded when snapshot was taken.

While using ShaderEffectSource, care is taken to minimize number of active pages by keeping them with visible: false as much as possible. Thus, the shader is not live, but takes snapshots on tabs overview activation. While cnapshot is taken, the page is set visible and visibility is removed after that.

However, turns out that without setting pages visible while loading, they don't show content correctly if made visible for short amount of time. Hence we keep them visible even if the page is not active tab. For that, inactive tabs are hidden behind the border and under other ones by reducing z.

As a result, all looks great on phone (as it should) and has been working for me.

Please review.