plug-ins: properly "Apply layer masks before saving" when checked.

The `if` test was inverted. When the option is checked, we don't want to
add the GIMP_EXPORT_CAN_HANDLE_LAYER_MASKS capability so that
gimp_export_image() applies the mask.
This is a first step to fix bug 783528, but it's not finished. Right
now, when the option is unchecked, the mask is simply ignored. We want
to export a PDF mask instead.