"Clear" stencil buffer manually; workaround for #5490 #5704
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As of Chrome v62, depth reads in certain Intel Iris graphics cards (at least Intel Iris Graphics 6100) all fail on intermittent frames, creating a flashing effect as in #5490. I believe this bug is some kind of race condition or incorrect vector access related to stencil buffer clearing. This PR mitigates this issue pending an upstream fix by never using
gl.clear(gl.STENCIL_BUFFER_BIT)
and instead "clearing" it manually by drawing a stencil mask of all 0s to the full viewport. I've confirmed on maps where this effect is most dramatic (maps with many sources) that this fix eliminates flashing entirely.Fixes #5490.
Upstream issue: https://bugs.chromium.org/p/chromium/issues/detail?id=784030