libgimp, plug-ins: massively improve GimpBrushSelectButton.

- Increase the default size to 40x40 and multiply it by the current window scale
  factor to have decent preview size.
- Make the brush preview always square with a GtkAspectFrame: even though
  brushes are not necessarily square, this is a much more obvious size rather
  than letting GTK choose a random allocation size which ends up very weird
  looking.
- Scale down the brush to the biggest possible dimensions which fit the square
  preview area (if the brush native size is already smaller, I don't scale up
  though) while keeping aspect ratio: previous implementation was really weird,
  as we were only seeing a tiny corner of much brushes as we weren't scaling
  them down. Obviously I use new gimp_brush_get_buffer|mask() functions for
  this as it supports scaling.
- Implement drawing color brushes too: the previous implementation was only
  drawing the brush mask, which was absolutely not what would be expected for
  such brushes.
- Add a white background behind color brushes with transparency.
- Simplify and clean up the code.

One of the consequences of this new implementation is obviously that it's
mandatory to call gegl_init() when using this widget.

plug-ins/python/test-dialog.py

@@ -18,6 +18,8 @@ gi.require_version('Gimp', '3.0')
 from gi.repository import Gimp
 gi.require_version('GimpUi', '3.0')
 from gi.repository import GimpUi
+gi.require_version('Gegl', '0.4')
+from gi.repository import Gegl
 from gi.repository import GObject
 from gi.repository import GLib
 from gi.repository import Gio
@@ -99,6 +101,7 @@ def test_dialog(procedure, run_mode, image, n_drawables, drawables, args, data):
 
     if run_mode == Gimp.RunMode.INTERACTIVE:
         GimpUi.init('python-fu-test-dialog')
+        Gegl.init(None)
         dialog = GimpUi.ProcedureDialog(procedure=procedure, config=config)
         dialog.fill(None)
         if not dialog.run():