Effect atlas

A UI with a large number of (usually small) effects often can't be effectively batched by the renderer. One approach to solving this might be API for ShaderEffectSource and/or item.layer, such that a set of source items are drawn to a common atlas.

For example:

Text {
    text: "Hello World"
    layer.enabled: true
    layer.atlas: "TextAtlas" // or reference to an Atlas item?
    layer.effect: ...
}

where the above would result in the text being rendered into a shared atlas, and that atlas becoming the source texture for the effect.