PLY: import/export custom vertex attributes (#108948)

Implements #108948 - support for custom point domain attributes
for PLY import and export. Notes:

- Custom attributes are always represented as scalar floats. PLY
  itself has some data types that blender can't fully represent, e.g.
  double or uint32, or some that fit into a float just fine but
  blender does not have them as separate types (e.g. int16).
- When importing, any PLY vertex attribute that is not one of
  "standard" names (position, normal, etc.) gets turned into a custom
  attribute.
- For exporting, more complex custom attributes (e.g. 2D/3D floats,
  quaternions, colors) get turned into several PLY attributes,
  with "_x" like suffix.

Custom attribute import/export is on by default in the UI.

@@ -150,0 +163,4 @@
+      return true;
+    }
+
+    const GVArraySpan attribute = *mesh->attributes().lookup(

@@ -150,0 +165,4 @@
+
+    const GVArraySpan attribute = *mesh->attributes().lookup(
+        attribute_id, meta_data.domain, meta_data.data_type);
+    if (attribute.is_empty()) {

@@ -150,0 +224,4 @@
+        r_attributes.append(attr_y);
+      } break;
+      case CD_PROP_FLOAT3: {
+        PlyCustomAttribute attr_x, attr_y, attr_z;

@@ -252,2 +252,4 @@
   }
 
+  Vector<int64_t> custom_attr_indices;
+  for (int64_t prop_idx = 0, n_props = element.properties.size(); prop_idx < n_props; ++prop_idx) {

@@ -331,1 +345,4 @@
     }
+
+    /* Custom attributes */
+    for (int64_t ci = 0; ci < custom_attr_indices.size(); ++ci) {

@@ -103,0 +103,4 @@
+  /* Custom attributes */
+  if (params.import_attributes && !data.vertex_custom_attr.is_empty()) {
+    for (const PlyCustomAttribute &attr : data.vertex_custom_attr) {
+      bke::SpanAttributeWriter<float> attr_writer =

@@ -17,1 +17,4 @@
 
+struct PlyCustomAttribute {
+  std::string name;
+  Vector<float> data; /* Any custom PLY attributes are converted to floats. */

@@ -150,0 +204,4 @@
+        }
+        r_attributes.append(attr_x);
+        r_attributes.append(attr_y);
+      } break;