USD: add USD importer

This is an initial implementation of a USD importer.

This work is comprised of Tangent Animation's open source USD importer,
combined with features @makowalski had implemented.

The design is very similar to the approach taken in the Alembic
importer. The core functionality resides in a collection of "reader"
classes, each of which is responsible for converting an instance of a
USD prim to the corresponding Blender Object representation.

The flow of control for the conversion can be followed in the
`import_startjob()` and `import_endjob()` functions in `usd_capi.cc`.
The `USDStageReader` class is responsible for traversing the USD stage
and instantiating the appropriate readers.

Reviewed By: sybren, HooglyBoogly

Differential Revision: https://developer.blender.org/D10700

2 files changed, 35 insertions, 0 deletions

diff --git a/source/blender/io/common/CMakeLists.txt b/source/blender/io/common/CMakeLists.txt
index 7e39af32f11..2aaf5d57fd6 100644
--- a/source/blender/io/common/CMakeLists.txt
+++ b/source/blender/io/common/CMakeLists.txt
@@ -37,6 +37,7 @@ set(SRC
 
   IO_abstract_hierarchy_iterator.h
   IO_dupli_persistent_id.hh
+  IO_types.h
   intern/dupli_parent_finder.hh
 )
 
diff --git a/source/blender/io/common/IO_types.h b/source/blender/io/common/IO_types.h
new file mode 100644
index 00000000000..4570e29f6ed
--- /dev/null
+++ b/source/blender/io/common/IO_types.h
@@ -0,0 +1,34 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2021 Blender Foundation.
+ * All rights reserved.
+ */
+#pragma once
+
+/* The CacheArchiveHandle struct is only used for anonymous pointers,
+ * to interface between C and C++ code. This is currently used
+ * to hide pointers to alembic ArchiveReader and USDStageReader. */
+struct CacheArchiveHandle {
+  int unused;
+};
+
+/* The CacheReader struct is only used for anonymous pointers,
+ * to interface between C and C++ code. This is currently used
+ * to hide pointers to AbcObjectReader and USDPrimReader
+ * (or subclasses thereof). */
+struct CacheReader {
+  int unused;
+};