[macosx 10.15] Fix GC resize issue #926 from @PetrBo and @tkittel

Name unnamed typedef struct {...} to fix MSVC error

Closing ZipArchive in destructor

osgsimplegl3: fix minor typo

[macosx 10.15] Fix for https://github.com/openscenegraph/OpenSceneGraph/issues/926 from @PetrBo and @tikittel

Compile with latest MSVC 16.6.0 Preview Release, got error message:
> osgPlugins\x\types.h(41,20): error C7626: unnamed class used in typedef name cannot declare members other than non-static data members, member enumerations, or member classes (compiling source file ...3rdparty\OpenSceneGraph\src\osgPlugins\x\mesh.cpp)

The fix was just to give it a name, which will never be used. I picked Vector_struct, feel free to improve.

Improve PkgConfig usage

Ignore Visual Studio Code configuration

png and tiff plugins do not depend on C++ exceptions.

Including FindPkgConfig directly into scripts is not a recommended practice.
Doing so will cause CMake to complain with a warning.
Replaced with find_package(PkgConfig QUIET)

fix bug in ProxyNode with DEFER_LOADING_TO_DATABASE_PAGER causing crash when children reorder during load.

ReaderWriterLAS recenter bug fixed, final batch of points was missed in translating the center point.

move Win32GWUtils header from src to include.

Fix for older versions of fbxsdk without xml or zlib libraries, fix cmake multiconfig generators (msvc) with irrelevant CMAKE_BUILD_TYPE.

Correct condition for copying openthreads PDB

Related to issue: https://github.com/openscenegraph/OpenSceneGraph/issues/898