Improvements for 2019 HT1

New features

• Select polygon mode from GUI, and simplify it
• Toggle visualisation of light cones in wireframe mode from the GUI
• Switch between shaders from GUI
• Replace lodepng with stb, to also support JPEG file loading
• Add shader for celestial rings and load it in lab1

Improvements

• Add a “CHANGES.rst” file that will list the different modifications done, from now on.
• Unify the TRS interfaces between the node and the TRSTransform classes, by using and exposing a TRSTransform instance inside the node class
• Edit node::render() to take parent transform
• AppVeyor: switch from VS 2019 Preview to VS 2019
• README: Add tinyfiledialogs to dependencies
• README: Sort the list of dependencies

Fixes

• Ensure Log is destroyed before its clients
• Rename WindowManager::CreateWindow() to WindowManager::CreateGLFWWindow(), to avoid conflict with Windows API macro.