[gimp] app: use gimp_drawable_supports_alpha() in GimpDrawableFilter

[Ell <ell src gnome org>]

commit 9947093d02f00376027b98349bf511397ddc16b2
Author: Ell <ell_se yahoo com>
Date:   Tue Apr 7 12:51:45 2020 +0300

    app: use gimp_drawable_supports_alpha() in GimpDrawableFilter
    
    ... instead of GIMP_IS_LAYER()

 app/core/gimpdrawablefilter.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/app/core/gimpdrawablefilter.c b/app/core/gimpdrawablefilter.c
index 0e2a755876..032b25d08f 100644
--- a/app/core/gimpdrawablefilter.c
+++ b/app/core/gimpdrawablefilter.c
@@ -891,8 +891,9 @@ gimp_drawable_filter_sync_format (GimpDrawableFilter *filter)
 {
   const Babl *format;
 
-  if (filter->add_alpha &&
-      (GIMP_IS_LAYER (filter->drawable) || filter->override_constraints))
+  if (filter->add_alpha                                &&
+      (gimp_drawable_supports_alpha (filter->drawable) ||
+       filter->override_constraints))
     {
       format = gimp_drawable_get_format_with_alpha (filter->drawable);
     }