这是一个枚举，指定 DrawBrushPreview 预览 PaintContext 的源渲染纹理还是目标渲染纹理。
| SourceRenderTexture | Specifies that TerrainPaintUtilityEditor.DrawBrushPreview uses the source render texture of the PaintContext. |
| DestinationRenderTexture | Specifies that TerrainPaintUtilityEditor.DrawBrushPreview uses the destination render texture of the PaintContext. |