Usd I/O: Basic dome light support, ArResolver support, and light scale support.

Removed options and functionality that can be submitted as
separate PRs.

@@ -1048,0 +1067,4 @@
+                  true,
+                  "Create Background Shader",
+                  "Convert first discovered USD dome lights to world background shader");
+

@@ -0,0 +218,4 @@
+        (bNodeSocketValueRGBA *)((bNodeSocket *)BLI_findlink(&fromnode->inputs, 0))->default_value;
+    bNodeSocketValueFloat *strength_data = (bNodeSocketValueFloat *)((bNodeSocket *)BLI_findlink(
+                                                                         &fromnode->inputs, 1))
+                                               ->default_value;

@@ -0,0 +321,4 @@
+    dome_light.CreateIntensityAttr().Set(res.world_intensity);
+  }
+
+  if (res.mapping_found) {

@@ -0,0 +365,4 @@
+  if (!scene->world->nodetree) {
+    scene->world->nodetree = bke::ntreeAddTree(NULL, "Shader Nodetree", "ShaderNodeTree");
+    if (!scene->world->nodetree) {
+      CLOG_WARN(&LOG, "Couldn't create world ntree");

@@ -0,0 +417,4 @@
+  }
+
+  /* Make sure the first input to the shader node is disconnected. */
+  bNodeSocket *shader_input = static_cast<bNodeSocket *>(BLI_findlink(&bgshader->inputs, 0));

@@ -227,2 +231,4 @@
     return new USDMeshReader(prim, params_, settings_);
   }
+  if (prim.IsA<pxr::UsdLuxDomeLight>()) {
+    /* We don't handle dome lights. */

@@ -43,1 +44,4 @@
 
+  /* USD dome lights are converted to a world material,
+   * rather than light objects, so are handled differently */
+  std::vector<pxr::UsdLuxDomeLight> dome_lights_;

@@ -112,6 +117,11 @@ class USDStageReader {
     return readers_;
   };
 
+  const std::vector<pxr::UsdLuxDomeLight> &dome_lights() const

@@ -285,0 +282,4 @@
+                            export_lights,
+                            export_cameras,
+                            export_curves,
+                            export_volumes};

@@ -316,6 +319,8 @@ static void wm_usd_export_draw(bContext *C, wmOperator *op)
   uiItemR(col, ptr, "export_normals", UI_ITEM_NONE, nullptr, ICON_NONE);
   uiItemR(col, ptr, "export_materials", UI_ITEM_NONE, nullptr, ICON_NONE);
   uiItemR(col, ptr, "export_custom_properties", UI_ITEM_NONE, nullptr, ICON_NONE);
+  uiItemR(col, ptr, "convert_world_material", UI_ITEM_NONE, nullptr, ICON_NONE);