在为 GraphicsTier.Tier2 编译着色器时，设置 UNITY_HARDWARE_TIER2。
A Shader only has per-tier variants if you use #pragma hardware_tier_variants or if the TierSettings for the current platform are different. This is because the tier settings impact the defines that Unity passes to the shader compiler. You can only set a GraphicsTier in the Built-in Render Pipeline.