Similarly, uses casts in `gtk_image_definition_unref()`. Compilation
succeeds without them, unlike in `gtk_image_definition_ref()`, because
`gtk_image_definition_unref()` is not called internally, unlike
`gtk_image_definition_ref()`. But the build would fail if some function
defined in the module were changed in the future to call
`gtk_image_definition_unref()`, and we can future-proof it now. This
also removes an inconsistency in how we access the members of the union.
`jhbuild build gtk+-3` with GCC 12 fails with:
```
In function ‘gtk_image_definition_ref’,
inlined from ‘gtk_image_definition_new_empty’
at ../../../../jhbuild/checkout/gtk+-3/gtk/gtkimagedefinition.c:107:10:
../../../../jhbuild/checkout/gtk+-3/gtk/gtkimagedefinition.c:241:13: error:
array subscript ‘GtkImageDefinition {aka union _GtkImageDefinition}[0]’
is partly outside array bounds of ‘GtkImageDefinitionEmpty[1]’
{aka ‘struct _GtkImageDefinitionEmpty[1]’} [-Werror=array-bounds]
241 | def->empty.ref_count++;
| ~~~~~~~~~~^~~~~~~~~~
../../../../jhbuild/checkout/gtk+-3/gtk/gtkimagedefinition.c:
In function ‘gtk_image_definition_new_empty’:
../../../../jhbuild/checkout/gtk+-3/gtk/gtkimagedefinition.c:105:34: note:
object ‘empty’ of size 8
105 | static GtkImageDefinitionEmpty empty = { GTK_IMAGE_EMPTY, 1 };
| ^~~~~
```
and many similar errors.
Fix it by casting `GtkImageDefinition` to `GtkImageDefinitionEmpty` or
similar, as appropriate.
Essentially backports 4dcd02e85315f487310e2e01fe9412706a77dc35 from main to gtk-3-24.
The size of icons is a property that is relevant to who is rendering the
icon, not to the icon itself.
Example: Starting a DND operation from an entry icon should cause the
icon to resize (from the entr icon's size to the DND icon size).
