GL3 backend: Set forward compatibility flag to fix running on macOS (… #1253

Job	Run time
Emscripten	4m 9s
macOS (-DSAMPLES_BACKEND=auto)	5m 50s
Windows (-DSAMPLES_BACKEND=auto -DENABLE_PRECOMPILED_HEADERS=OFF)	8m 54s
macOS (-DSAMPLES_BACKEND=GLFW_GL2)	4m 13s
Windows (-DSAMPLES_BACKEND=Win32_VK -DRMLUI_VK_DEBUG=ON)	9m 2s
Linux (clang, clang++, -DENABLE_PRECOMPILED_HEADERS=OFF -DSAMPLES_BACKEND=GLFW_GL2)	2m 32s
Windows (-DSAMPLES_BACKEND=SDL_VK -DBUILD_LUA_BINDINGS_FOR_LUAJIT=ON)	7m 55s
Linux (clang, clang++, -DSAMPLES_BACKEND=SDL_VK)	1m 54s
Linux (-DBUILD_TESTING=ON -DENABLE_PRECOMPILED_HEADERS=OFF, true)	3m 26s
Linux (-DNO_FONT_INTERFACE_DEFAULT=ON -DENABLE_LOTTIE_PLUGIN=ON -DSAMPLES_BACKEND=X11_GL2)	2m 11s
Linux (-DDISABLE_RTTI_AND_EXCEPTIONS=ON -DSAMPLES_BACKEND=SDL_GL2)	2m 40s
Linux (-DNO_THIRDPARTY_CONTAINERS=ON -DSAMPLES_BACKEND=SFML_GL2)	2m 2s
Linux (-DSAMPLES_BACKEND=SDL_VK -DRMLUI_VK_DEBUG=ON -DENABLE_PRECOMPILED_HEADERS=OFF -DCMAKE_BUIL...	2m 24s
	57m 12s

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