add test for bounds clipping edge case

tests/data/good_maps/bounds_clipping.xml

@@ -0,0 +1,47 @@
+<!-- without maximum-extent this will drop features if a 
+     buffer_size is used of >= 128 which can be simulated with:
+     
+a tile request like: /2/0/1.png
+
+or using nik2img:
+
+nik2img.py tests/data/good_maps/bounds_clipping.xml t.png -e -20037508.34 -5009377.085697313 -5009377.08569731 15028131.25709193 -d 256 256
+
+no buffer can be simulated with:
+
+nik2img.py tests/data/good_maps/bounds_clipping.xml t.png -d 256 256 -e -20037508.342789244 0 -10018754.171394622 10018754.17139462
+
++proj=krovak +lat_0=49.5 +lon_0=24.83333333333333 +alpha=30.28813975277778 +k=0.9999 +x_0=0 +y_0=0 +ellps=bessel +units=m +towgs84=498.17,136.89,510.08,6.007,4.343,3.831,3.38 +no_defs
+
+-->
+
+<Map 
+  srs="+init=epsg:900913" 
+  minimum-version="2.0.0" 
+  buffer-size="128"
+  maximum-extent="-20037508.34,-20037508.34,20037508.34,20037508.34"
+>
+
+  <Style name="style">
+     <Rule>
+        <PolygonSymbolizer fill="darkred"/>
+     </Rule>
+    
+   </Style>
+
+  <!-- layer in standard mercator: being protected into 
+       spherical mercator makes it prone to feature loss
+       when using map buffer
+  -->
+  <Layer 
+      name="countries" 
+      srs="+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs"
+  >
+    <StyleName>style</StyleName>
+    <Datasource>
+      <Parameter name="type">shape</Parameter>
+      <Parameter name="file">../shp/world_merc2.shp</Parameter>
+    </Datasource>
+  </Layer>
+
+</Map>