vg_common_json: Support mask with matte case.

vg_common_json: Support mask with matte case.

Summary:
The layer can have both a mask node and a matte node.
In this case, one of them may be missing because it uses one mask_set api.
If there is a matte, the mask is to be child of the matte.

Test Plan:
.._mask_set(layer, matte);
.._mask_set(layer, mask);

Reviewers: Hermet, kimcinoo

Reviewed By: Hermet

Subscribers: cedric, #reviewers, smohanty, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8709

Change-Id: I2a617cbaf087ae9839befa40b94d6e40e2d859ce