Implement UIs using RmlUi

vendor/README.md

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+# Vendor dependencies
+
+TODO: This should be automated with shuriken at some point. For now, manual instructions.
+
+
+## System dependencies
+
+For now, SDL3 and SDL3_image need to be installed on the system. They should be vendored at some point.
+
+Same for freetype, which is required by RmlUi.
+
+
+## RmlUi
+
+[RmlUi](https://github.com/mikke89/RmlUi) is a UI library.
+
+Within the `vendor` directory, run:
+
+```shell
+# Either clone git repository or download and unpack archive (version 6.2)
+git clone git@github.com:mikke89/RmlUi.git RmlUi --branch 6.2
+cd RmlUi
+
+# Configure and build with cmake (use samples or standalone preset)
+cmake -G Ninja -B Build --preset samples -DBUILD_SHARED_LIBS=false -DRMLUI_BACKEND=SDL_SDLrenderer
+cmake --build Build
+
+# Optionally test by running a sample
+./Build/rmlui_sample_demo
+```