Fix #109070: Creating mesh from Python skips setting faces sharp

The default value of the "sharp_face" attribute is False like other
boolean attributes. That mistakenly changed behavior in addons
that created meshes with `Mesh.from_pydata`. To fix, add an
argument with a default value that maintains the behavior
from before, and add convenience, `shade_smooth` and
`shade_flat` methods.

scripts/modules/bpy_types.py

@@ -534,7 +534,7 @@ def ord_ind(i1, i2):
 class Mesh(bpy_types.ID):
     __slots__ = ()
 
-    def from_pydata(self, vertices, edges, faces):
+    def from_pydata(self, vertices, edges, faces, shade_flat=True):
         """
         Make a mesh from a list of vertices/edges/faces
         Until we have a nicer way to make geometry, use this.
@@ -591,6 +591,9 @@ class Mesh(bpy_types.ID):
         self.polygons.foreach_set("loop_start", loop_starts)
         self.polygons.foreach_set("vertices", vertex_indices)
 
+        if shade_flat:
+            self.shade_flat()
+
         if edges_len or faces_len:
             self.update(
                 # Needed to either:
@@ -605,6 +608,26 @@ class Mesh(bpy_types.ID):
     def edge_keys(self):
         return [ed.key for ed in self.edges]
 
+    def shade_flat(self):
+        """
+        Render and display faces uniform, using face normals,
+        setting the "sharp_face" attribute true for every face
+        """
+        sharp_faces = self.attributes.new("sharp_face", 'BOOLEAN', 'FACE')
+        for value in sharp_faces.data:
+            value.value = True
+
+    def shade_smooth(self):
+        """
+        Render and display faces smooth, using interpolated vertex normals,
+        removing the "sharp_face" attribute
+        """
+        try:
+            attribute = self.attributes["sharp_face"]
+            self.attributes.remove(attribute)
+        except KeyError:
+            pass
+
 
 class MeshEdge(StructRNA):
     __slots__ = ()