Avoid Writing WMS Requests To Disk

[warmerdam]

It would be desirable to avoid writing WMS requests fetched for WMS client layers to disk to avoid the associated performance costs.

[warmerdam]

I have implemented this in trunk (r10540).

Note that setting the metadata "wms_cache_to_disk" to "true" or "1" will give the old behavior of writing to disk which can be valuable to take advantage of caching.

No specific tests written though msautotest/gdal/wmsclient*.map passing is a good sign.

A similar approach could be applied to WFS client layers, but that is beyond the scope of my customers needs.

[warmerdam]

Documentation updated (r10543). Also other minor fixes (r10541,r10542).

[dmorissette]

I noticed that the new code uses GDAL VSI* calls to open the image in memory, essentially adding a GDAL dependency on mapwmslayer.c which was not there in 5.6. (I have verified that mapwmslayer.c doesn't build if you configure --with-wmsclient --with-proj --with-curl --without-gdal and it used to build in 5.6)

I'm okay with the new GDAL dependency for mapwmslayer.c personally, GDAL was already recommended in previous releases anyway in order to support reprojection of cascaded requests, so I have updated the configure script to check this dependency and the README.CONFIGURE to document it in r10548.