use st_force2d instead of deprecated st_force_2d (#4803)
use version specific postgis force2d function (#4803)

also adds a PROCESSING "FORCE2D=no" to avoid using force2d altogether (#4024)
Allows floating point NoData/NULLVALUEs for floating point images.
Merge PR #4803
fix travis build
Merge branch 'branch-6-4' into fa-6-4

BUGZID:
Properly find the tclgd library.  tcl.gd needs to create a libtclgd.so symlink to the actual shared library.  It's a TEA-standard build, though.
BUGZID:
BUGZID:
Perform the msSetup() call during Mapscript Tcl initialization to set up the font cache and threads if so configured.
BUGZID:
BUGZID: