gt-cql: add build-helper-maven-plugin
[GEOT-4824] Integrating unsupported's gt-mark-wkt into gt-render
 GEOT-4830 add fallback mark to SLD Graphic render

This fix applies to any 'failed' ExternalGraphics (unreachable at the
provided URI) and prevents nothing from being rendered as well as many
(one per feature) log messages and attempts to resolve the URI.

Additionally, the failed attempt is logged (once per render) as a
warning and contains the URI.
Merge pull request #489 from ischneider/GEOT-4380

 GEOT-4830 add fallback mark to SLD Graphic render
Merge pull request #485 from srstclair/gt-cql-build-helper

gt-cql: add build-helper-maven-plugin
Merge pull request #483 from n-lagomarsini/GEOT-4824

[GEOT-4824] Integrating unsupported's gt-mark-wkt into gt-render