stencil separate support

**Is your feature request related to a problem? Please describe.**
```c++
glStencilFunc( GL_ALWAYS, 0, 0xff );
glStencilOpSeparate( GL_FRONT, GL_KEEP, GL_INCR_WRAP, GL_INCR_WRAP );
glStencilOpSeparate( GL_BACK, GL_KEEP, GL_DECR_WRAP, GL_DECR_WRAP );
glDrawArrays( GL_TRIANGLES, 0, fcount );
```
https://github.com/astiopin/sdf_atlas/blob/2158930242546df5827df75418ef5d046c10b97d/src/sdf_gl.cpp#L112

As shown above sdf rendering requires stencil buffer op. Currently filament only support stencil in OpenGL backend. And users can't configure front or back buffer separately.

**Describe the solution you'd like**
It will be nice to export some fields in `.mat` file for stencil op.

**Describe alternatives you've considered**
No.

**OS and backend**
No.