add partial test files for second case of #997

tests/data/good_maps/raster-alpha-gradient.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE Map[]>
+<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over" background-color="transparent">
+
+<Style name="white" filter-mode="first">
+  <Rule>
+    <RasterSymbolizer opacity="1" scaling="bilinear" />
+  </Rule>
+</Style>
+<Layer name="white"
+  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
+    <StyleName>white</StyleName>
+    <Datasource>
+       <Parameter name="file">../raster/transp.tiff</Parameter>
+       <Parameter name="type">gdal</Parameter>
+    </Datasource>
+  </Layer>
+
+</Map>