Evas masking: Fix rendering of masks that belong to a proxied smart object

Evas masking: Fix rendering of masks that belong to a proxied smart object

This is a complex situation:
- Smart object A contains image I
- A is proxied into an image B
- B is marked as source_invisible, which means A is invisible
- Mask M is applied to image I
- Mask M is ALSO a smart child of A
Because of all that, mask M could not be rendered into its private
mask surface, as it was falling under the case of "parent_src_invisible".

This patch checks that the object is not a mask during the
parent_src_invisible check.

@fix