evas: Implement support for external buffers

evas: Implement support for external buffers

This brings support for the eo api for external buffers (like
the old data_set / data_get). The new API now works with slices
and planes.

The internal code still relies on the old cs.data array for
YUV color conversion. This makes the code a little bit too
complex to my taste.

Tested with expedite for RGBA and YUV 422 601 planar, both
SW and GL engines (x11).