VAR_LIGHT_MAP_UV语义
这个变量见于：https://catlikecoding.com/unity/tutorials/custom-srp/baked-light/
解释在：https://catlikecoding.com/unity/tutorials/custom-srp/draw-calls/
we need to pass the coordinates to the fragment function as it is there that the texture is sampled.
so add float2 baseUV to Varyings as well. this time we do not need to add a special meaning, it is just data that we pass along that does not require special attention from the GPU. however, we still have to attach some meaning to it.
we could apply any unused identifier, let us simply use VAR_BASE_UV.
所以后文你可以看到诸如：
这里只有一个SV_POSITION有系统值意义，SV，是system value的缩写。
https://docs.microsoft.com/zh-cn/windows/win32/direct3dhlsl/dx-graphics-hlsl-semantics?redirectedfrom=MSDN
