Evas filters: fix random cases of 'dancing text'

Evas filters: fix random cases of 'dancing text'

In some situations, text with filters would be rendered in an invalid
position (somewhere too high).
I am not entirely sure of the reason why the original code with BLEND
doesn't work, but this new version is simpler as GL and SW have more
similar behaviours:

- render text to our 'output' buffer
- draw this buffer as an image onto the set target

Thanks zmike for reporting the issue.
And thanks A LOT for using the filters :D

@fix

Signed-off-by: Jean-Philippe Andre <jp.andre@samsung.com>