Evas masking: Fix masking of multiple objects in a map

Evas masking: Fix masking of multiple objects in a map

All those masking bug fixes become harder to explain. But here goes:

 - Take a genlist, apply a mask to it (for example put everything
   in an elm_layout). Also play with various objects in the genlist.
 - Also apply a map to it (for instance, elm_transit zoom).

 --> Now some elements will be masked, some others will not,
     and some may even not render at all.

This patch restores a mask in the current drawing context, instead
of just unsetting it.