Specifies a Render Texture to render the panel's UI on.
This can be used to display UI on a 3D geometry in the Scene, to perform manual UI post-processing, or
render the UI on a MSAA-enabled Render Texture.
When the project color space is linear, you should use a Render Texture whose format is
GraphicsFormat.R8G8B8A8_SRGB.
When the project color space is gamma, you should use a Render Texture whose format is
GraphicsFormat.R8G8B8A8_UNorm.