USD: Explicit default values for exported mesh attributes

Mesh attributes make up the USD mesh data, so contain the vertices,
loops, face vertex counts, etc. By explicitly passing default values for
those, the USD library will only write those values once when they don't
change from frame to frame.

USD: updated GTest to use the BlendfileLoadingBaseTest introduced in D6246

Merge branch 'wip-ctest-blenloader' into sybren-usd

Fixed memory leak

Deinitialisation of Blender

Added GPL header to each new file

Blendfile-loading test class

To test some functionality of the USD exporter, I want to be able to load a blend file and run parts of the exporter. Up to now this wasn't possible from a GTest test. Of course a Python test could run the USD exporter, but testing the result would require writing a parser for USD files, which I want to avoid.

This new test class minimally sets up Blender so that it can load blend files and construct a depsgraph without crashing. Note that I haven't tested this on very complex blend files, so it may still crash when the loaded blend file references/requires uninitialised data structures.

The test will certainly crash with Blend files created with Blender 2.80, as the versioning code requires space types to be registered. This is normally done by initialising the window manager, which is not done in this test. The WM requires Python to run, which in turn requires that Blender finds the release directory in the same directory that contains the running executable, which is not the case for GTest tests (they are written to `bin/tests/executablename`.

This patch requires the functionality from {D6236}.

Differential Revision: https://developer.blender.org/D6246

Updated license blocks

FIXUP for EXTRA_CLI

CMake/GTest: allow defining CLI arguments for GTEST tests

For USD tests I need to load a blend file from a GTest test, and in order to do that properly it needs to get the location of the lib/tests directory. The easiest way to do this is to pass it on the CLI.

To make it possible to extend the `BLENDER_SRC_GTEST_EX()` without adding yet more macros, it now accepts keyword arguments.

Differential Revision: https://developer.blender.org/D6236

Header guard #define

Updated for review comments

- Removed link to CMake docs
- EXTRA_CLI ? COMMAND_ARGS
- BlendfileLoadingAbstractTest ? BlendfileLoadingBaseTest
- Moved BlendfileLoadingBaseTest into tests/gtests/blenloader
- Added failure message
- Check C++ string with a C++ string function
- Full stop.

More feedback from Sergey

USD: Marked `USDHierarchyIterator::stage` as `const`

No functional changes.

USD: Made all fields in `USDExporterContext` const

No functional changes.

USD: No longer unpack USDExporterContext struct

The `USDExporterContext` struct contents were duplicated in the
`USDAbstractWriter` member properties, a decision which in retrospect I
don't agree with.

No functional changes.

USD: added extra comments and reordered functions

Added extra comments to explain more of the structure of the
`AbstractHierarchyIterator`, and reordered function declarations in the
header file so that it's easier to understand. Also reordered the code
in the CC file so that it has the same order as the declarations.

No functional changes.

USD: Removed unused function writer_map()

USD: Created abstract Blendfile-loading test class

This test class minimally sets up Blender so that it can load blend files
and construct a depsgraph without crashing. Note that I haven't tested this
on very complex blend files, so it may still crash when the loaded blend
file references/requires uninitialised data structures.

I also changed the USD Export Hierarchy Iterator Test to use the new test
class, and renamed it to Abstract Hierarchy Iterator Test as it doesn't
test anything USD-specific.

Merge remote-tracking branch 'origin/master' into sybren-usd