Unexpected background of sdf icon when icon-halo-width more than some critical value.

mapbox-gl-js version: 0.48.0

browser: chrome

Steps to Trigger Behavior

1. Add sdf icon
2. Set icon-halo-width to some big value

Link to Demonstration

http://jsbin.com/heqadegucu/edit?html,output

Expected Behavior

We should not see an unexpected background of image. (In this example icon-halo-width has a value less than critical)

Actual Behavior

We can see an unexpected background of image.

Additional information

For some combination of values, everything is fine (e.g. icon-halo-width=2, icon-halo-blur=2, these are the same values as used in [icon-halo-blur/literal| https://github.com/mapbox/mapbox-gl-js/blob/8121e9db044533d7e44e0afc8c58e71b6d8ec260/test/integration/render-tests/icon-halo-blur/literal/style.json] test, also OK for 3/1, 3/2, 4/1 combinations of icon-halo-width, icon-halo-blur).
But for some combination of values of icon-halo-width/icon-halo-blur (e.g. 3/3, 4/2, 4/3, 4/4, 5/1, 6/0) there’s a semi-transparent fill for whole icon background. It seem to have different transparency depending on particular values, for some values the fill is almost invisible, for others it is hard to miss it even with the naked eye.
This seems like a bug, which makes icon halo unusable for some cases.
It could be reproduced using SDF icon (e.g. https://github.com/mapbox/mapbox-gl-js/blob/8121e9db044533d7e44e0afc8c58e71b6d8ec260/test/integration/image/line.sdf.png) or sprites (like in https://github.com/mapbox/mapbox-gl-js/blob/8121e9db044533d7e44e0afc8c58e71b6d8ec260/test/integration/render-tests/icon-halo-blur/literal/style.json test). The same effect happens in different browsers (Chrome, Firefox, Internet Explorer)