Additional tile layers flicker after zoom animation

[mourner]

When you zoom in, the animation goes fine with several layers, but in the end, after the first layer is loaded, other layers scaled from previous zoom disappear under it, which is seen as an annoying layer flickering.

[cgalli]

@mourner, on iOS I can't seem to find a zoom animation glitch with multiple layers. The link I sent in #123 illustrates 4 tile layers (two WMS) and it all animates fine no matter how aggressive I pinch/pan, etc.I just update to newest code this morning, so didn't test when this issue was opened. Perhaps fixed for iOS? Android suffers from major bug #32 so no testing there yet.

[mourner]

Renamed the issue to be more descriptive. It's about the case where you zoom in, the animation goes fine with several layers, but in the end, after the first layer is loaded, other layers scaled from previous zoom disappear under it, which is seen as an annoying layer flickering.

To illustrate, suppose we have 2 layers, when we zoom in the order is like this:
Old zoom layer 1
Old zoom layer 2
New zoom layer 1
New zoom layer 2

But to look nice it should be like this:
Old zoom layer 1
New zoom layer 1
Old zoom layer 2
New zoom layer 2

[fredleefarr]

Have just experienced this issue introducing multiple layers using 0.3.1.

[mourner]

A difficult issue to solve, postponed to 0.5

[mourner]

Will be fixed by #1140