[gimp] plug-ins: try to clarify "save-transp-pixels" argument of tiff saving.

[Jehan <jehanp src gnome org>]

commit ae8bf15758da4a874d3d684a0a4d6bdf6e9ad1a0
Author: Jehan <jehan girinstud io>
Date:   Mon Mar 11 13:04:36 2019 +0100

    plug-ins: try to clarify "save-transp-pixels" argument of tiff saving.
    
    This argument should actually say "Do not store premultiplied channel
    values", which is what the TIFF spec calls "Unassociated alpha" (vs.
    "associated alpha" when values are stored premultiplied by alpha).
    
    Now I can see where the current description is coming from, which is
    that any color with alpha 0 (totally masked) ends up as RGBA value (0,
    0, 0, 0), in other words, the color information is completely lost. Yet
    this label is not very helpful to understand what the checkbox really
    does. I decided to not just change it altogether as people would have
    gotten used to this for years, but at least adding completary
    information in API and tooltip in GUI.
    
    (copied from 18a2f576bbd9e1be155886b8da206f282f9053e0 instead of
    cherry-picking since differences in the ui file was making
    cherry-picking over-complicated for just one line)

 plug-ins/file-tiff/file-tiff.c   | 2 +-
 plug-ins/ui/plug-in-file-tiff.ui | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/plug-ins/file-tiff/file-tiff.c b/plug-ins/file-tiff/file-tiff.c
index 94a81136b4..3c50a00567 100644
--- a/plug-ins/file-tiff/file-tiff.c
+++ b/plug-ins/file-tiff/file-tiff.c
@@ -129,7 +129,7 @@ query (void)
   static const GimpParamDef save_args[] =
   {
     COMMON_SAVE_ARGS,
-    { GIMP_PDB_INT32, "save-transp-pixels", "Keep the color data masked by an alpha channel intact" }
+    { GIMP_PDB_INT32, "save-transp-pixels", "Keep the color data masked by an alpha channel intact (do not 
store premultiplied components)" }
   };
 
   gimp_install_procedure (LOAD_PROC,
diff --git a/plug-ins/ui/plug-in-file-tiff.ui b/plug-ins/ui/plug-in-file-tiff.ui
index 888829eabc..4b017cc730 100644
--- a/plug-ins/ui/plug-in-file-tiff.ui
+++ b/plug-ins/ui/plug-in-file-tiff.ui
@@ -33,6 +33,7 @@
     <child>
       <object class="GtkCheckButton" id="save-alpha">
         <property name="label" translatable="yes">Save color values from transparent pixels</property>
+        <property name="tooltip_text" translatable="yes">Colors are not stored premultiplied by the 
associated alpha</property>
         <property name="visible">True</property>
         <property name="can_focus">True</property>
         <property name="receives_default">False</property>