Add hooks into the render pipeline

[vicapow]

Would love to be able to add our own custom webGL overlays between existing layers. something like this maybe?

map.renderHookAfterLayer('before-this-layer', function renderCallback(gl) {
  // gl calls.
});

Specifically, this would allow us to add custom overlays like heatmaps, etc. with labels on top and geo below.

[vicapow]

very similar to: #281 maybe a duplicate?

[enjalot]

+1
don' think this is a duplicate, overlay would be different from something like being able to run a desaturation filter on a raster layer which needs #281 I believe.

[lucaswoj]

IF we consider this feature, it'll be after we have a bulletproof solution for #145.

[lucaswoj]

I'm going to call this a duplicate of #281 and re-open that issue.