Add unit test for the `<paint-area>` of decorators #1564

Job	Run time
macOS (-DRMLUI_BACKEND=auto)	1m 34s
macOS (-DRMLUI_BACKEND=GLFW_GL2)	1m 30s
Windows (-DRMLUI_BACKEND=auto -DRMLUI_PRECOMPILED_HEADERS=OFF)	5m 29s
Linux-legacy (clang, clang++, -DRMLUI_BACKEND=GLFW_GL2 -DCMAKE_BUILD_TYPE=Debug)	1m 29s
Emscripten	7m 17s
Linux-legacy (clang, clang++, -DRMLUI_BACKEND=SDL_VK -DBUILD_TESTING=ON -DRMLUI_PRECOMPILED_HEADE...	2m 13s
Linux (clang, clang++, -DRMLUI_BACKEND=GLFW_GL2 -DRMLUI_PRECOMPILED_HEADERS=OFF)	1m 56s
Windows (-DRMLUI_BACKEND=Win32_VK -DRMLUI_VK_DEBUG=ON)	4m 52s
Linux-legacy (-DBUILD_TESTING=ON)	1m 35s
Linux (clang, clang++, -DRMLUI_BACKEND=SDL_VK -DCMAKE_BUILD_TYPE=Debug -DBUILD_TESTING=ON)	1m 44s
MinGW64	5m 22s
Windows (-DRMLUI_BACKEND=SDL_VK -DRMLUI_LUA_BINDINGS_LIBRARY=luajit)	4m 49s
Linux-legacy (-DRMLUI_BACKEND=native -DCMAKE_BUILD_TYPE=Debug -DRMLUI_VK_DEBUG=ON)	1m 37s
Linux (-DRMLUI_BACKEND=GLFW_GL3 -DBUILD_TESTING=ON, true)	3m 19s
Linux (-DRMLUI_BACKEND=X11_GL2 -DRMLUI_LOTTIE_PLUGIN=ON)	1m 9s
Linux (-DRMLUI_BACKEND=SDL_GL2 -DRMLUI_CUSTOM_RTTI=ON -DCMAKE_CXX_FLAGS="-fno-exceptions -fno-rtti")	1m 14s
Linux (-DRMLUI_BACKEND=SFML_GL2 -DRMLUI_THIRDPARTY_CONTAINERS=OFF)	1m 13s
Linux (-DRMLUI_BACKEND=GLFW_VK -DCMAKE_BUILD_TYPE=Debug -DRMLUI_VK_DEBUG=ON -DRMLUI_PRECOMPILED_H...	1m 41s
	50m 3s

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