[raster] Make an aggregate version of SameAlignment

[pracine]

So that we can verify quickly if all the raster in a table (or a selection) have the same alignment. Should return false if one of the raster is not aligned.

This could be useful to any process aggregating raster (like ST_Union and the GDAL driver in mode=2) to make sure of the right alignment of every raster involved in the aggregation. If one raster is not aligned these process should warn the users that he has to resample some of the raster or that he should pass some alignment parameters if the process accept some.

[dustymugs]

A plpgsql version has been committed in r9953. Might consider a C version if performance is lacking.