plug-ins: fix #9067 TGA files saved by Krita can't be opened

Krita had a bug where it incorrectly saved 4 for the alpha bits instead of
8. We will allow 4 and convert that to 8 to be able to read the incorrect
TGA images.

The likelyhood of real TGA images with 4 alphabits seems pretty low, so it
should not interfere with real images that have this set. We were not
supporting that anyway.

plug-ins/common/file-tga.c

@@ -556,6 +556,11 @@ load_image (GFile   *file,
       if (info.imageType == TGA_TYPE_GRAY && info.bpp == 16)
         info.alphaBits = 8;
     }
+  else if (info.alphaBits == 4 && info.imageType == TGA_TYPE_COLOR && info.bpp == 32)
+    {
+      /* Incorrect TGA saved by Krita, see issue #9067*/
+      info.alphaBits = 8;
+    }
 
   switch (info.imageType)
     {