This provides better usability when a GradientTexture or CurveTexture
is added to a Control node.
Visual appearance of most GradientTextures and CurveTextures will
be unaffected.
Neither name is a perfect match but `Curve3Texture` looked too similar to
`CurveTexture` and `Curve3D`, which made things confusing when picking a
texture type or browsing the API reference.