[wktraster] Implement ST_GetBBox function for raster

[mloskot]

Function ST_GetBBox is part of Objective 0.1.6e and shall be implemented:

ST_GetBBox(raster) -> polygon geometry

In the original presentation  WKTRasterSpecifications1.0.ppt, slides 31, 32, 33 present what ST_AsPolygon(), ST_Shape(), ST_GetBBOX() and ST_Envelop() should return.

Functions ST_GetBBOX(), ST_Envelop() and ST_Shape() are necessary to implement an elementary version of ST_Intersect() and ST_AsPolygon() is necessary to implement an elementary version of ST_Intersection() returning geometries only.

We definitely need to first write the complete specs of these functions in order to agree on what they are supposed to do. Feedback welcome. Please, post your comments below.

[pracine]

ST_GetBBox(raster) is replaced with ST_Envelope() since there is no more an equivalent ST_GetBBox function in PostGIS.

See the working specifications?.