IGL: Move virtual method implementation to .h file #333

Job	Run time
Android (Ubuntu)	16m 9s
cmake-build (Windows - MSVC 2022, windows-latest, Debug, cl, cl, Visual Studio 17 2022, -DIGL_WIT...	8m 50s
cmake-build (Windows - MSVC 2022 (no OpenGL), windows-latest, Debug, cl, cl, Visual Studio 17 202...	7m 16s
cmake-build (Ubuntu - Clang, ubuntu-latest, Debug, clang, clang++, Unix Makefiles, -DIGL_WITH_TRA...	7m 39s
cmake-build (Ubuntu - GCC, ubuntu-latest, Debug, gcc, g++, Unix Makefiles, -DIGL_WITH_TRACY=ON -D...	9m 25s
cmake-build (macOS - Clang (Xcode), macos-latest, Debug, clang, clang++, Xcode, -DIGL_WITH_VULKAN...	7m 58s
cmake-build (macOS - Clang (Unix Makefiles), macos-latest, Debug, clang, clang++, Unix Makefiles,...	8m 6s
	1h 5m 23s

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