Grease Pencil: Convert from mesh to Grease Pencil

Convert mesh objects to grease pencil, optionally preserve mesh faces as
filled shapes, and convert crease edges into strokes. Following options
are available:

- Whether to convert faces to filled strokes.
- Stroke thickness.
- Stroke offset (Using normal direction to lift strokes out of mesh
  surfaces).

Note that "Crease Angle" option from legacy grease pencil is not ported
to this implementation. This option is deemed more suitable for using
geometry nodes to achieve.

Resolves #126480

Pull Request: https://projects.blender.org/blender/blender/pulls/131854

tests/python/object_conversion.py

@@ -136,10 +136,10 @@ def main():
         ConversionPair('Curves 6', 'BezierCircle', 'CURVES', 'CURVES'),
         ConversionPair('Curves 7', 'BezierCurve', 'CURVES', 'CURVES'),
         ConversionPair('Curves 8', 'Text', 'CURVES', 'CURVES'),
-        ConversionPair('GreasePencil 1', 'Cube', 'GREASEPENCIL', 'MESH'),
-        ConversionPair('GreasePencil 2', 'CubeWithEdges', 'GREASEPENCIL', 'MESH'),
-        ConversionPair('GreasePencil 3', 'Plane', 'GREASEPENCIL', 'MESH'),
-        ConversionPair('GreasePencil 4', 'HollowPlane', 'GREASEPENCIL', 'MESH'),
+        ConversionPair('GreasePencil 1', 'Cube', 'GREASEPENCIL', 'GREASEPENCIL'),
+        ConversionPair('GreasePencil 2', 'CubeWithEdges', 'GREASEPENCIL', 'GREASEPENCIL'),
+        ConversionPair('GreasePencil 3', 'Plane', 'GREASEPENCIL', 'GREASEPENCIL'),
+        ConversionPair('GreasePencil 4', 'HollowPlane', 'GREASEPENCIL', 'GREASEPENCIL'),
         ConversionPair('GreasePencil 5', 'Suzanne', 'GREASEPENCIL', 'GREASEPENCIL'),
         ConversionPair('GreasePencil 6', 'BezierCircle', 'GREASEPENCIL', 'GREASEPENCIL'),
         ConversionPair('GreasePencil 7', 'BezierCurve', 'GREASEPENCIL', 'GREASEPENCIL'),