_Time是个4维向量，跟Unity3D中的deltaTime（这是个一维的，数值）不同。
float4 _Time : Time (t/20, t, t*2, t*3), use to animate things inside the shaders。
fixed x= Speed * _Time; 等价于 fixed x= Speed * _Time.x;
| Time | float4 | Time (t/20, t, t*2, t*3), use to animate things inside the shaders. |
| _SinTime | float4 | Sine of time: (t/8, t/4, t/2, t). |
| _CosTime | float4 | Cosine of time: (t/8, t/4, t/2, t). |
| unity_DeltaTime | float4 | Delta time: (dt, 1/dt, smoothDt, 1/smoothDt). |
| _ProjectionParams | float4 | x is 1.0 (or –1.0 if currently rendering with a flipped projection matrix), y is the camera’s near plane, z is the camera’s far plane and w is 1/FarPlane. |
| _ScreenParams | float4 | x is the current render target width in pixels, y is the current render target height in pixels, z is 1.0 + 1.0/width and w is 1.0 + 1.0/height. |
参考： https://docs.unity3d.com/462/Documentation/Manual/SL-BuiltinValues.html
