Workaround for failure to clear depth buffer on some Android devices.

[ChrisLoer]

Fixes issue #3437.

We don't expect gl.depthRange to affect the behavior of gl.clearDepth, but experimentally, it appears that we're failing to clear the depth buffer on some Android devices, and resetting the depth range before the clear call appears to fix the problem (maybe because some drivers implement clearDepth in terms of just drawing a full screen quad to the depth buffer?). This only happens once per frame, so should be negligibly expensive.

The only place I could reproduce this issue was on a BrowserStack Galaxy S6 image running Chrome 68.0.3440.85/Android 5.0.2 -- we'll need to collect feedback from other people who have encountered #3437 to see how full a fix this is.

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/studio and/or @mapbox/maps-design if this PR includes style spec changes

/cc @ansis @mollymerp @chloekraw

[ansis]

It might be worth checking whether mapbox-gl-native has any issues that could be solved by this fix.

[ChrisLoer]

@tobrun wasn't aware of any Android rendering issues that sounded like this one. Initial feedback from #3437 is that this fix is working, and I also confirmed on the BrowserStack Note 4 image... so I think this makes sense to merge, even though I'd still like to learn more about what's going on.