GetSupportedCRSNames should filter out the CRS without validated bounding box

[yangm]

The spatial context collection contains all the supported CS and boundingbox for each CS. But the bounding box for each CS is the union of each small bounding box from different layer which also support the same CS.

But this will cause a defect for the following situation: The root layer support CS: CS-1, CS-1, but without specific boundingbox for each CS. There is child layer: Layer1, supporting CS-1, with its small bounding box; child layer: Layer2, supportting CS-2, with its smail bounding box;

For the above example, the supported CS list for layer2 should only have CS-2, because there is no valid bounding box for CS-1 though the CS-1 is supported by it from WMS standard.