[raster] Make a ST_Extent(raster) aggregate taking rotation into account

[pracine]

Right now we can use ST_Extent(rast::geometry) to get the non rotated extent of a set of rasters but we might want to get the rotated extend instead.

This aggregate should be based on a _st_extent(raster, raster) doing basically the samething as ST_MapAlgebra(_{,'UNION') when it precompute the size of the resulting raster.}

This could then be used by the GDAL driver or any process needing to precalculate the size of a coverage before burning tiles in it.

[pracine]

Could or should be called ST_RotatedExtent instead...

[dustymugs]

Isn't this just a union on the set of convex hulls?

[pracine]

No because the union does not necessarily result in a nice rectangle if some tiles are missing. here we want the same kind of object as the result of ST_Extent but rotated.

[dustymugs]

So, 5 vertices in the polygon. Call ST_ConvexHull(geometry) with the final union...

[dustymugs]

Actually, ST_Extent(rast::geometry) returns the correct (rotated if raster is rotated). The raster cast to geometry uses ST_ConvexHull().