Override default layer styles to allow animations inside shader

Nobody has claimed this yet.

## Motivation

I want to be able to create a spinner-like circle and an animated arrow like line using shader (for performance reasons).
I did that with mapbox gl js versions before 0.40 with a shader hack (I just replaced the circle fragment shader and sending performance.now() as a new u_time variable in the shader). However, I would like to know if it would be possible to do this correctly with the new versions.

### Implementation

We just need to send performance.now() or RequestAnimationFrame value to all the default style layers. Then there should be a simple way to simply provide a custom fragment.glsl file to override the default ones.

I know we could do the same with the new custom layers, but that would be overkill to create a renderer from scratch just to animate a circle or a line.

[See this gist for a working fragment shader](https://gist.github.com/kaligrafy/d2fa2155a70940f08170b13436bd5515)

Start by reviewing the existing custom-layer approach and the working fragment shader linked in the issue. Define how default style layers would receive animation timing and how users would provide replacement fragment shaders; done means both capabilities work without requiring a renderer from scratch.

Written by the indexing model from the issue text.