Remove face selection for a PreviewPass

Fixes issue where the selected face for the lay-flat feature showed
up in the model thumbail of print jobs.

CURA-6754

cura/PreviewPass.py

@@ -64,6 +64,7 @@ class PreviewPass(RenderPass):
                 self._shader.setUniformValue("u_ambientColor", [0.1, 0.1, 0.1, 1.0])
                 self._shader.setUniformValue("u_specularColor", [0.6, 0.6, 0.6, 1.0])
                 self._shader.setUniformValue("u_shininess", 20.0)
+                self._shader.setUniformValue("u_faceId", -1)  # de-select any selected faces
 
         if not self._non_printing_shader:
             if self._non_printing_shader: