plug-ins: fix #1111 TGA with indexed colors + alpha channel : not displayed correctly

Just like COLOR targa MAPPED (indexed) images with a size of 32 per
index should also have alphaBits set to 8 or else the images get
loaded incorrectly.

plug-ins/common/file-tga.c

@@ -547,6 +547,9 @@ load_image (GFile   *file,
   /* hack to handle yet another flavor of incorrect headers, see bug #540969 */
   if (info.alphaBits == 0)
     {
+      if (info.imageType == TGA_TYPE_MAPPED && info.colorMapSize == 32)
+        info.alphaBits = 8;
+
       if (info.imageType == TGA_TYPE_COLOR && info.bpp == 32)
         info.alphaBits = 8;