Clamped billboards can shake

Noticed during review of #4715, clamped billboards, label glyphs, and point primitives can have a 1-pixel horizontal shake when rotating the camera. This can be seen by loading the Ground Clamping Sandcastle demo, especially by setting a dark label background:


It looks like maybe the horizontal position is being rounded incorrectly, allowing rounding errors to shake by a whole pixel.

Start by loading the Ground Clamping Sandcastle demo and reproducing the 1-pixel horizontal shake while rotating the camera, especially with a dark label background. Trace the horizontal positioning and rounding for clamped billboards, label glyphs, and point primitives. Done means these primitives remain visually stable during camera rotation.

Written by the indexing model from the issue text.