Implement clip masks for the GL2 renderer #1220

Job	Run time
macOS (-DSAMPLES_BACKEND=auto)	2m 39s
Linux (clang, clang++, -DENABLE_PRECOMPILED_HEADERS=OFF -DSAMPLES_BACKEND=GLFW_GL2)	42s
macOS (-DSAMPLES_BACKEND=GLFW_GL2)	2m 41s
Emscripten	2m 14s
Windows (-DSAMPLES_BACKEND=auto -DENABLE_PRECOMPILED_HEADERS=OFF)	8m 29s
Linux (clang, clang++, -DSAMPLES_BACKEND=SDL_VK)	1m 45s
Windows (-DSAMPLES_BACKEND=Win32_VK -DRMLUI_VK_DEBUG=ON)	9m 48s
Linux (-DBUILD_TESTING=ON -DENABLE_PRECOMPILED_HEADERS=OFF, true)	1m 57s
Windows (-DSAMPLES_BACKEND=SDL_VK -DBUILD_LUA_BINDINGS_FOR_LUAJIT=ON)	9m 48s
Linux (-DNO_FONT_INTERFACE_DEFAULT=ON -DENABLE_LOTTIE_PLUGIN=ON -DSAMPLES_BACKEND=X11_GL2)	1m 27s
Linux (-DDISABLE_RTTI_AND_EXCEPTIONS=ON -DSAMPLES_BACKEND=SDL_GL2)	1m 47s
Linux (-DNO_THIRDPARTY_CONTAINERS=ON -DSAMPLES_BACKEND=SFML_GL2)	39s
Linux (-DSAMPLES_BACKEND=SDL_VK -DRMLUI_VK_DEBUG=ON -DENABLE_PRECOMPILED_HEADERS=OFF -DCMAKE_BUIL...	3m 41s
	47m 37s

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