evas_vg_load_svg: Support multiple gradient without <defs> part

evas_vg_load_svg: Support multiple gradient without <defs> part

Summary:
Multiple gradients can be declared.
There is a problem of keeping only one gradient information
when it is declared outside defs or when defs is not declared.
It supports the use of multiple gradients even if no defs are declared.

Test Plan:
(with D9312 patch)
cd src/example/edje
edje_cc -beta svg.edc && gcc -o svg-test svg-test.c `pkg-config --libs --cflags evas ecore ecore-evas edje`
./svg-test

Reviewers: Hermet, kimcinoo, smohanty

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9315

Change-Id: I22fda2ab7ae0eb01d6923ebaf134aee54114de1a