[Xastir] Latest OSM tweaks

[Curt, WE7U]

Jerry sent me some OSM tweaks Friday.  I managed to get them
tested and committed to CVS.  Below are Jerry's notes about the
patch.  I personally did a Reindex New on the maps a couple of
times, then killed/restarted Xastir to get the *.osm maps to be
removed from Map Chooser.  Perhaps one reindex would have worked,
but I didn't check the map index file until I had done it twice.

Read Jerry's notes below.  BTW:  The <F4> key seems to work well for
me.  I'm using FVWM2 like Tom is.

Thanks again Jerry, looks good!  --Curt.

---------------------------------------------------------------------

Summary of changes:
   - scaling corrected
   - requested OSM map size limited to 2000x2000 and +/-89 deg latitude
   - no 'binned' zoom levels
   - new function key (F4) to 'optimize' zoom level for OSM
   - .osm maps eliminated
   - OSM map files (.geo) built from a single template

Details:
   scaling corrected:
     This required a major rewrite of the map_OSM.c code since
     the map_tiger.c code that it was based on did not support
     Mercator projection.

     Note that due to size limitations from the server and the
     use of Mercator scaling when the OSM bitmap images are rendered,
     some OSM maps will not fill the display window. This is particularly
     common when using a large display window. Selecting an optimized OSM
     zoom level (F4) will generally, but not always, result in bit map
     image that fills the window.

   map size limitations:
     The OSM map server that is used by this code limits the returned
     bitmap image to 2000 pixels in either direction. Also, as a side
     effect of the Mercator projection, latitude values must be limited
     to -90 < lat < 90.

   no binned zoom levels:
     Attempts to constrain the linear Xastir scaling to the binned OSM
     levels has been unsuccessful. The final straw was a problem with
     float to integer transitions that would result in looping and
     oscillation between levels.

   new function key to optimize OSM zoom levels:
     Since bitmap images do not scale well, they will always look best when
     the Xastir scale is closest to the OSM scale. A function key (F4) has
     been defined that will adjust the Xastir scale to the nearest OSM level
     and redraw the screen. Note that even if the scale is not changed, the
     F4 key will redraw the screen- this is intentional on my part.

     The F4 key works only if an OSM map is selected.

     If the F4 key is already defined for some other use on your system you
     can specify an alternative key by modifying the map files (OSM*.geo).
     The specified value must be an X11 KeySym value. Setting the value to 0
     will disable the key.

   .osm maps eliminated
     Since binned scale levels are no longer supported, the .osm extension
     is no longer needed. .osm file will not be installed.

     If your maps directory (typically /usr/local/share/xastir/maps/Online/)
     contains .osm file from an previous install you will have to remove
     them by hand.

     You should also re-index your files (Map->Configure->Index:Reindex
     ALL maps) to remove them from the Map Chooser.

   OSM map files (.geo) built from a single template:
     This is primarily of interest to developers and results in a cleaner
     source tree.