Billboard sizeInMeters clamped to 65 kibi-meters

[Sandcastle example](https://sandcastle.cesium.com/#c=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) -- the purple border should line up with the edges of the teal fill, and it does for the smaller shape (44km) but not the larger one (88km).

Per [discussion on the forum](https://community.cesium.com/t/what-on-earth-is-going-on-with-billboardgraphics-sizeinmeters/10260/2?u=james_b) it looks like at some point the billboard width is being clamped to 65535. I'm guessing height has the same problem. The clamping should be applied to the actual pixel count of the texture, not the logical size.

Start with the linked Sandcastle example and compare the 44 km and 88 km billboards, then trace how BillboardGraphics sizeInMeters is converted into texture and pixel dimensions. Check the forum discussion for the reported 65535 clamp; done means the purple border aligns with the teal fill for both sizes, including the corresponding height behavior.

Written by the indexing model from the issue text.