Add new custom shader parameter to style

[ibesora]

Motivation

The current approach to shaders doesn't allow the user to create maps with custom-encoded data loaded via raster or vector tiles.
Take for example a map where the raster data is normal-mapped or something like this where the tiles loaded encode temperature or precipitation.

Design Alternatives

Do nothing and solve this case forcing the users to load ready-to-show raster or vector data, removing the extra-processing from the client and into the server, but losing the possibility of animatinc such data.

Design

Something like the GLTF technique's definition added to the style definition and new attributes to define shaders and which techniques are used would work.

Mock-Up

style = { "sources": { "normals": { "type": "raster", ... "technique": "normalMapping" } }, ... "techniques": { "normalMapping": { "program": "program0", ... } }, "programs": { "program0": { "fragmentShader": "duck0FS", "vertexShader": "duck0VS" }, ... } };

Implementation

The style parser should be modified to process the new tags and create the new techniques.
There should be a way to send uniforms to the shader per layer from the API so they can be changed on each repaint.
An error popup should be shown when the shader is not supported by the hardware it's tried to be run on.

[jfirebaugh]

Thanks for the suggestions. We're tracking a very similar feature request in #281; let's keep discussion centralized there.

[andrewharvey]

or something like this where the tiles loaded encode temperature or precipitation.

and for simple colour mapping where you don't need a custom shader, it should be addressed by #3889