Stepped heatmap colors still have gradient on edges with low intensity

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**mapbox-gl-js version**: 0.52

**browser**: chrome

### Steps to Trigger Behavior

1. Make a heatmap and set the `heatmap-color` to `'step'` instead of `'interpolate'`
2. Set the `heatmap-intensity` to `0.01`
3. Make the first `step` trigger at `0.001` (so that 0 is still transparent but any value at all will tip it over into having a color)

### Link to Demonstration

https://jsfiddle.net/strangerelics/Lf93vq5w/30/

### Expected Behavior

The outer edges of the heatmap are hard lines with no gradient, just like the inner edges.

### Actual Behavior

The outer edges of the heatmap are a gradient out to transparent, even though the inner edges are hard lines.

Start with the linked JSFiddle and reproduce the issue using heatmap-color set to step, heatmap-intensity 0.01, and a first step at 0.001. Compare the expected hard outer edge with the observed fade to transparency, then trace the heatmap rendering path until the differing edge behavior is located; done means the outer and inner step edges render consistently.

Written by the indexing model from the issue text.