Make sampleHeight work for point clouds

[lilleyse]

sampleHeight, sampleHeightMostDetailed, and others will usually pick through a point cloud despite requesting the high detail tiles. localhost:8080 Example

One idea to fix this is render to a larger than 1x1 canvas with attenuation turned on.

[lilleyse]

This can actually be fixed in a much simpler way by making the ray's frustum width larger.

Centimeter frustum:

Meter frustum:

Five meter frustum:

localhost:8080 example using #7287