CustomLayerInterface to expose projection matrix parameters

Nobody has claimed this yet.

## Motivation

For our [custom layer implementation](https://deck.gl/docs/api-reference/mapbox/overview) to properly match the base map in an interleaved 3D context, we need access to the projection matrix parameters.

`CustomLayerInterface.render` currently exposes a single `matrix` argument that is the combined view projection matrix. In our implementation, coordinate transforms are done on the GPU for better performance, but shader uniforms only support up to 32-bit floats. To compensate for lost precision, we calculate the view matrix and projection matrix separately.

Historically we poke into the source code of the `Transform` class to try match our calculation with the base map. This has become increasingly problematic as the base map could change its near/far plane calculation between versions. It would be much easier if `CustomLayerInterface.render` could expose the view and projection matrices separately, or even the individual projection matrix parameters (fov, near, far).

## Design Alternatives

A non-breaking API change could be adding a third parameter to `CustomLayerInterface.render`:

```js
render(gl: WebGLRenderingContext, matrix: number[], transform: {
pixelMatrix: number[];
projMatrix: number[];
...
})
```

### Implementation

The separate matrices and nearZ/farZ etc. are already stored in `map.transform` as private properties. Making them visible to a custom layer has minimal overhead but will help create much more robust third-party renderers.

Start by locating CustomLayerInterface.render and the map.transform data mentioned in the issue, then trace how the existing combined matrix is supplied. Define the non-breaking API shape, expose the needed separate matrices or projection parameters, and verify that existing custom layers remain compatible.

Written by the indexing model from the issue text.