[gimp/gimp-2-10] plug-ins: add support for grayscale images in psp reader.

[Jehan <jehanp src gnome org>]

commit 9ec44fd835ee806c11c807b2031e378f271de6a9
Author: Jacob Boerema <jgboerema gmail com>
Date:   Tue Sep 8 22:29:45 2020 -0400

    plug-ins: add support for grayscale images in psp reader.
    
    (cherry picked from commit 2054f9f1634480890cf457d9037eff064bae100c)

 plug-ins/common/file-psp.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)
---
diff --git a/plug-ins/common/file-psp.c b/plug-ins/common/file-psp.c
index 1f1244f230..726324503c 100644
--- a/plug-ins/common/file-psp.c
+++ b/plug-ins/common/file-psp.c
@@ -935,10 +935,10 @@ read_general_image_attribute_block (FILE     *f,
     else
       ia->bytes_per_sample = 1;
 
-  if (ia->base_type != GIMP_RGB)
+  if (ia->base_type == GIMP_INDEXED)
     {
       g_set_error (error, G_FILE_ERROR, G_FILE_ERROR_FAILED,
-                   _("Unsupported bit depth %d"), ia->depth);
+                   _("Indexed images are not supported"));
       return -1;
     }
 
@@ -1778,16 +1778,21 @@ read_layer_block (FILE      *f,
           null_layer = TRUE;
         }
 
-      if (ia->grayscale)
-        if (!null_layer && bitmap_count == 1)
+      if (ia->base_type == GIMP_RGB)
+        if (bitmap_count == 1)
+          drawable_type = GIMP_RGB_IMAGE, bytespp = 3;
+        else
+          drawable_type = GIMP_RGBA_IMAGE, bytespp = 4;
+      else if (ia->base_type == GIMP_GRAY)
+        if (bitmap_count == 1)
           drawable_type = GIMP_GRAY_IMAGE, bytespp = 1;
         else
-          drawable_type = GIMP_GRAYA_IMAGE, bytespp = 1;
+          drawable_type = GIMP_GRAYA_IMAGE, bytespp = 2;
       else
-        if (!null_layer && bitmap_count == 1)
-          drawable_type = GIMP_RGB_IMAGE, bytespp = 3;
+        if (bitmap_count == 1)
+          drawable_type = GIMP_INDEXED_IMAGE, bytespp = 1;
         else
-          drawable_type = GIMP_RGBA_IMAGE, bytespp = 4;
+          drawable_type = GIMP_INDEXEDA_IMAGE, bytespp = 2;
       bytespp *= ia->bytes_per_sample;
 
       layer_ID = gimp_layer_new (image_ID, layer_name,