replace polyvox with simpler implementation

[RTUnreal]

This replaces the functionality of the polyvox library, so we don't have to ship it anymore. :)

This will to introduce a bit of a breaking change, where if borders are filled in, then the faces at the border is not rendered. But considering, that polyvox already did it at some of the sides, I tought it would be better to always not render faces there, so that tiling can be done in the future. This can be changed back if needed

[ksuprynowicz]

Thank you! Everything looks good :)
I only have minor feedback about some of the formatting changes, let me know what you think about it.

[ksuprynowicz]

Thank you! Everything looks good :)

[ksuprynowicz]

I'm getting an assert in Overte_hub:

[02/20 16:13:01] [FATAL] [default] ASSERT: "isInside(index)" in file /home/ksuprynowicz/overte/overte_opt/overte/libraries/entities-renderer/src/RenderablePolyVoxEntityItem.cpp, line 192

[ksuprynowicz]

Backtrace:

__pthread_kill_implementation 0x00007fffec29e95c
__pthread_kill_internal 0x00007fffec29e9ff
__GI_raise 0x00007fffec249cc2
__GI_abort 0x00007fffec2324ac
QMessageLogger::fatal(char const*, ...) const 0x00007ffff6a92514
qt_assert(const char *, const char *, int) 0x00007ffff6a915ae
VoxelVolume::getVoxelAt RenderablePolyVoxEntityItem.cpp:192
operator() RenderablePolyVoxEntityItem.cpp:614
loop3<…>(const glm::vec<…> &, const glm::vec<…> &, struct {...}) RenderablePolyVoxEntityItem.cpp:153
CubicSurfaceExtractorWithNormals::createMesh RenderablePolyVoxEntityItem.cpp:603
operator() RenderablePolyVoxEntityItem.cpp:1933
ReadWriteLockable::withReadLock<…>(struct {...} &&) const ReadWriteLockable.h:109
operator() RenderablePolyVoxEntityItem.cpp:1918
QtConcurrent::StoredFunctorCall0::runFunctor(void) qtconcurrentstoredfunctioncall.h:70
QtConcurrent::RunFunctionTask::run qtconcurrentrunbase.h:142
<unknown> 0x00007ffff6ae5e21
<unknown> 0x00007ffff6ae28fd
start_thread 0x00007fffec29cb7b
__GI___clone3 0x00007fffec31a7b8