The Openraster plug-in calls file-png-load and
gimp-file-load-layer but didn't check if the result
was SUCCESS. Even though I haven't seen any
failures let's improve error checking by adding
a check for a correct result.
The current openraster specification only has a limited
number of layer modes. Let's improve things a little bit
by mapping both the current and legacy layer modes
to the same openraster composite-op where available.
For the other layer modes we could examine what the
closest composite-op is, or add a GIMP specific op
(which is what Krita seems to do). For now we set
all these to the openraster equivalent of NORMAL.
Let's use the non legacy layer modes when importing/exporting
openraster images. That way we are more likely to be able to
import the correct layer modes since openraster only has a
limited number of layer modes defined.
This fixes#5838 which was caused by an incomplete port
of this plug-in to the new introspection API.
For now we do keep the n_drawables parameter of save_ora until
issue #5312 is fixed. Not doing this would cause saving to fail.
Plug-ins that work from different bindings probably want to use their
own list-type to specify arguments, rather than working with a more
cumbersome `GimpValueArray`.
This new API should make it less verbose. For example:
```
args = Gimp.ValueArray.new(5)
args.insert(0, GObject.Value(Gimp.RunMode, Gimp.RunMode.NONINTERACTIVE))
args.insert(1, GObject.Value(Gimp.Image, image))
args.insert(2, GObject.Value(Gimp.Drawable, mask))
args.insert(3, GObject.Value(GObject.TYPE_INT, int(time.time())))
args.insert(4, GObject.Value(GObject.TYPE_DOUBLE, turbulence))
Gimp.get_pdb().run_procedure('plug-in-plasma', args)
```
becomes
```
Gimp.get_pdb().run_procedure('plug-in-plasma', [
GObject.Value(Gimp.RunMode, Gimp.RunMode.NONINTERACTIVE),
GObject.Value(Gimp.Image, image),
GObject.Value(Gimp.Drawable, mask),
GObject.Value(GObject.TYPE_INT, int(time.time())),
GObject.Value(GObject.TYPE_DOUBLE, turbulence),
])
```
Apart from porting, only code logics change is the whole
encode()/decode() code because it created a string vs bytes mess and the
zipfile API apparently didn't like to deal with bytes, even though the
docs say otherwise.
It's hard to test on my UTF-8 system, so please anyone working with
non-UTF-8 paths, we welcome reports if ORA load/save does not work
properly.
Other than this, load, save and load_thumb were all tested and working
properly so far.
