网址：Making multiple shader program variants
How multi_compile works
一般的
- 每一个都是一个 variant，如果没有开启任何一个，默认启用第一个
特殊的
- 对于所有的都是以“__”开头的，则默认创建一个空的，避免另外创建一个关键词
Difference between shader_feature and multi_compile
- shader_feature 等价于 multi_compile __
- shader_feature 可以在面板上设置，multi_compile 通过代码来设置
Built-in multi_compile shortcuts
- multi_compile_fwdbase compiles all variants needed by ForwardBase (forward rendering base) pass type. The variants deal with different lightmap types and main directional light having shadows on or off.
- multi_compile_fwdadd compiles variants for ForwardAdd (forward rendering additive) pass type. This compiles variants to handle directional, spot or point light types, and their variants with cookie textures.
- multi_compile_fwdadd_fullshadows - same as above, but also includes ability for the lights to have realtime shadows.
- multi_compile_fog expands to several variants to handle different fog types (off/linear/exp/exp2).
