Evas filters: Implement proxy rendering for GL

Evas filters: Implement proxy rendering for GL

Make use of glReadPixel to access the source's pixel data.
Use all classic CPU functions to blend and use that data.
Save pointer to the GL image and update it with the latest data
during target render.

Use ENFN's surface_lock, read_pixels, unlock.

Also, add some more error checks to make sure the images are valid,
or return an error at runtime.