Reproduces a dumb situation with Unity surface shaders where you try to use `_MainTex_ST` and find it is undefined.
Then when you try to define it, you get told that it's a duplicate definition.

This appears to be ["intended behaviour"](https://forum.unity.com/threads/mixed-vert-frag-surf-shader-error-redefinition-_maintex_st.502940/#post-9049588).

The attempted workaround, however, doesn't work for me. It compiles fine, but I just get black for the UVs.

The actual, viable workaround is to name the variables so they don't start with `uv_` (so that Unity doesn't stomp them for you), and then populate them from a custom vertex shader.