Merge tag '6.6.1-release' of https://github.com/secondlife/viewer into heimdall

doc/contributions.txt

indra/cmake/CMakeLists.txt

indra/cmake/FindZLIBNG.cmake

indra/cmake/LLCommon.cmake

indra/cmake/LLPrimitive.cmake

indra/cmake/ZLIBNG.cmake

indra/doxygen/CMakeLists.txt

indra/llappearance/lltexlayer.cpp

indra/llappearance/lltexlayer.h

indra/llcommon/CMakeLists.txt

indra/llcommon/classic_callback.cpp

indra/llcommon/classic_callback.h

indra/llcommon/llsdserialize.cpp

indra/llcommon/llsys.cpp

indra/llcommon/llsys.h

indra/llcommon/tests/classic_callback_test.cpp

indra/llcorehttp/CMakeLists.txt

indra/llimage/CMakeLists.txt

indra/llinventory/llparcel.cpp

indra/llplugin/llpluginclassmedia.h