Mesh Gradient not copied into other document
#### Steps to reproduce:
- open Inkscape
- open file
- copy the rectangle (mesh gradient filled)
- open new file (ctrl+n)
- paste

#### What happened?
Fill info (mesh gradient) lost
...
#### What should have happened?
It should have been copied.
...
#### Inkscape Version and Operating System:
- Inkscape Version: 1.0beta1 (fe3e306, 2019-09-17)
- Operating System: Windows 10
- Operating System version: 1903 (Build 18362.356)
[2019.12.08_Mesh_Gradient_Imitating_Interpolate.svg](/uploads/4f91bc0da12c4d6512aa3fddb77d702d/2019.12.08_Mesh_Gradient_Imitating_Interpolate.svg)
<!--
**Thank you for filling in a new bug report!**
*More details on how to write a good bug report can be found at https://inkscape.org/contribute/report-bugs/.
Please remember that Inkscape is developed mostly by volunteers in their spare time, and may not be able to respond to reports quickly.*
-->
issue