[WMS Client] set styles parameter when sending remote WMS GetMap requests

[tomkralidis]

As a result of #1088, STYLES is a required parameter in MapServer WMS server requests.

mapwmslayer.c must now:

- check for wms_style in .map file - if wms_style is set, i.e. "wms_style" "foostyle"

• look for wms_foostyle_sld
• if wms_foostyle_sld is set
  • send "STYLES=foostyle&SLD=wms_foostyle_sld" as part of the WMS request
• if wms_foostyle is not set
  • send "STYLES=foostyle" as part of the WMS request

- if wms_style is not set

• send "STYLES=" as part of the WMS request

We should get this in for 5.0 given #1088.

Changes:

• source code:
• documentation: no change, if noone sets a style "STYLES=" is set
• msautotest: there are no WMS client tests

[tomkralidis]

Looking at mapwmslayer.c (line 308 ish), the code already did most this. But STYLES= was only being passed if the pszStyle wasn't empty. Now fixed in r6616. Updated HISTORY.TXT.