Require Validation of SLD ExternalGraphic OnlineResource (#4883)

Onlineresource must validate against "sld_external_graphic" entry of
map->web->validation in order to be taken into account.

added thanks to Regione Toscana - SITA
Fix SDE plugin build (#4887)
Merge pull request #4891 from szekerest/sdefix

Fix SDE plugin build (#4887)
Rename predefined field names for cluster and union layers (#4082)
Merge branch 'master' of https://github.com/mapserver/mapserver into issue4082
Merge pull request #4894 from szekerest/issue4082

Issue4082
add RFC103 layer encoding setter/getter to swig (#4758)
99c0e8b is not needed (#4758)
const-correctness

Add "const" to a bunch of pointers that are not written to.
mapxbase: make DBFIsValueNULL() static

It's an internal function.
simplify SfRealloc()

realloc(NULL,size) is equivalent to malloc(size).  No need for this
runtime check.
remove NULL checks before free()

free(NULL) is legal (no-op).
mapfile.c: don't redefine _GNU_SOURCE

Avoids compiler warning.
Fix dateline wrapping logic for bounding boxes (#4896)

- Fixes a typo in the shapefile wrapping code to compare a vertex
  against its predecessor rather than the initial vertex to detect
  potential dateline hops.
- Forces +over proj parameter when reprojecting bboxes in order to
  conserve the initial property of wether the bbox intersects the
  dateline.
Allow AGG images bigger than 2 GB (#4421)