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

[Jacob Boerema <jboerema src gnome org>]

commit a77131db3c52f77bf854da0dd81b2ef40faf0556
Author: Jacob Boerema <jgboerema gmail com>
Date:   Fri Oct 15 11:43:34 2021 -0400

    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 | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/plug-ins/common/file-tga.c b/plug-ins/common/file-tga.c
index 09aab84f9f..8a74ecc51e 100644
--- a/plug-ins/common/file-tga.c
+++ b/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;