PG12: Optimize FunctionCall parameter size in rtpg_mapalgebra.c

[Algunenano]

In #4313 we adapted the code to support PostgreSQL 12's new way of handling FunctionCallInfoData parameters. It went for always allocating space for FUNC_MAX_ARGS to an on-demand request.

The initial approach taken was to keep the old behaviour and request space for FUNC_MAX_ARGS parameters, but that can be optimized and request memory only for the amount of parameters used per function (typically 2-4).