How to get tile's extent by function requestImage in construct imageryProvider

I resloved some format ‘.mvt’ files have data polygon and attributes by openlayers’s format/mvt.js, and then i tried to use them in cesium, with rebuild imageryProvider it did worked. It shows data correctly in cesium viewer, but be rendered into image, all i want is use these data create polygon entity directly,can some one help me?
data case:
image
image
image

cood:mecator84,
canvas:512 512,
extent:0 0 4096 4096