SDF label pixel ratio inconsistency

[IanLilleyT]

Discussion started here: #8351 (comment)

The SDF label looks different at 1.0 vs 2.0 pixel ratio:

But with a czm_pixelRatio "fix" in BillboardCollectionFS.glsl, they look the same:

However, this removes the crispness on normal labels:
1.0 PR:

2.0 PR

2.0 PR with pixelRatio "fix"

I'm not sure which of these approaches to use. The underlying problem is shader derivatives. The smoothing is based off the fwidth of the texture's distance value and the sample offset is based of the dFdx and dFdy of the texture coordinates. For pixelRatio 2.0 these values are effectively half of pixelRatio 1.0, resulting in the cripser image. Might require a deeper dive into SDF labels at some point.