ST_Collect is an aggregation function for aggregating geometry data. It
can be used as a group aggregate function and also as a window aggregate
function.

ST_Collect aggregates geometries from a column having geometry data,
returning Geometrycollection, Multipoint, Multilinestring, Multipolygon
or NULL.

Change-Id: I5a6c486279e899b3387b3c21b92b03d3722f90f3