evas vg: improve masking render logic.

evas vg: improve masking render logic.

Previous masking image is generated on vg rendering time,
though context is changed to main vg rendering to masking,
This had a issue that ector context switching which is not allowed
in software backend because of asynchronou method.

Now, this improvment brings the masking rendering move to
vg render pre step with synchronous rendering method,
then use the masking surface in the main vg rendering.

Change-Id: I59b4c14899a9c3cd794273bf46e3c286c15561a3