Fix: USD: Wrong UsdUVTexture rgb output type.

This fixes a bug where the rgb UsdUVTexture shader output
attributes were created with incorrect type color3f when
connected to UsdPreviewSurface diffuseColor and emissiveColor
inputs. The UsdPreviewSurface specification requires the
UsdUVTexture rgb output to be of type float3.

This was happening because the UsdUVTexture outputs were created
implicitly by the call to UsdShadeInput::ConnectToSource().  I.e.,
because the diffuseColor and emissiveColor inputs are of type
color3f, this was the type assigned by default to the rgb source
attributes as well.

Now explicitly creating the UsdUVTexture shader output attributes
with the correct types.

Pull Request: https://projects.blender.org/blender/blender/pulls/114728