Fit texture coordinates to rectangle/trapezoid geometry

It would be useful to support stretching a texture along a rectangle/trapezoid geometry, or perhaps even support explicit texture coordinates. The effect would look like the second image:

@rumicuna created a demo for testing this:

https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Hello%20World.html&label=Showcases&gist=079b7bbc6c2c16bdaf23c57fe0a7f2c4

Start with the linked Sandcastle Hello World demo and inspect the geometry and texture-coordinate path it exercises. The issue leaves the API choice open between stretching textures over rectangle/trapezoid geometry and accepting explicit texture coordinates, so review the 17-comment discussion before deciding scope. Done means the requested texture mapping is supported and demonstrated by the effect shown in the issue images.

Written by the indexing model from the issue text.