evas: Fix support for image_data_get on snapshot

evas: Fix support for image_data_get on snapshot

evas_object_image_data_get() is a legacy API that I made work
with snapshot objects (evas_object_image_snapshot_set()). Some
changes in the engine broke the behaviour and this patch fixes
it.

When getting the pixels from an FBO, in read-only mode, we need
to create a temporary image (pixels surface) that contains the
copy of the pixels we get from glReadPixels. This image needs
to be deleted afterwards. It is thus stored by the image object
and freed upon _image_data_set() (good) or object deletion (bad).

FBO + read-write is not supported by this API (it is supported
through buffer_map as the filters had to use that).

Fixes T5754