[SLD] support other symbols beside the weelknow ones

[assefa]

The idea is to support other symbol names inside the wllKnownName tag.
It is not in the spec and I am not how to adress this limitation.  
The idea is that a symbol name that is not a well know could be mapped to an
existing symbol in the sym file. If the symbol is not found, the default symbol
(sqaure) would be used.

[assefa]

Initial discussions with Tom, Bart and Tim seems to indicate that It would be a
usuful addition without breaking any other rules.

The idea is the following : if a symbol name is used and is not a weel know
symbol, we check if the symbol exists in the symbol list attached to the map. If
it exists, we use it, if not the default symbol square is used.

Will send a note to the mapserver user list to give a chance for others to comment.

[tomkralidis]

added myself to cc.  Email snippet below:

SLD 1.0.0 (https://portal.opengeospatial.org/files/?artifact_id=1188) says:

(Section 11.3.2):
The WellKnownName element gives the well-known name of the shape of the mark.
Allowed values include at 
least “square”, “circle”, “triangle”, “star”, “cross”,
and “x”, though map servers may draw a different symbol instead if they don't 
have a shape for all of these. The default WellKnownName is “square”.  
Renderings of these marks may be made solid or hollow depending on Fill and 
Stroke elements. These elements are discussed in Sections 11.2.2 and 11.1.3, 
respectively.

So MapServer w/ SLD support should support the 6 default symbol types.

Suggested behaviour:

- client passes SLD with Mark using an alternate WellKnownName, like "mysymbol"
- MapServer SLD support checks for this symbol def in the mapfile's symbol defs
- if it exists, it is used
- if it does not exist, defaults to "square"

[assefa]

Commited the changes. Will wait for additnal tests before closing. My test were
using a tag like this <WellKnownName>tent</WellKnownName> where I had the symbol
'tent' defined in my symbol file.

[tomkralidis]

Tested.  Works.  Thanks.