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 18a2f576bb 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

@@ -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,

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>