As mentioned in issue #1777, exporting non-animated WebP images was
only keeping the current layer.
Mimick file-gif-save.c: display the encoding settings pop-up earlier
so that gimp_export_image() can merge the layers unless "As Animation"
is enabled. Call gimp_image_get_layers() directly in save_image() in
case the layers were merged (for clarity because layers are used only
for animations).
(cherry picked from commit 8f828d1899)
