Evas filters: Final fix after the ector refactor

Evas filters: Final fix after the ector refactor

This implements a generic way of scaling buffers, using fake
RGBA_Image wrapping ector buffer maps. The underlying algo is
still the good old linear sw scaler.

Now the filters *should* be back to their previous level of
usability. Performance will probably be even worse than it was
before, for GL, as more glReadPixels may be involved. Optimization
now consists in actually implementing the filters with GL shaders.