textpath: improves text rendering quality of curved text

textpath: improves text rendering quality of curved text

There was wrong logic for calculating # of slices, dt, dist of each segment.
It caused bad rendering quality by putting too much slices on small text.

In addition, textpath didn't care about smoothness of curve's slope changing.
The patch fixes to check differences of previous points and next points for Evas Map.
So, textpath can show smoothly curved text.

- Patch on EFL's upsteam:
https://phab.enlightenment.org/D6195

@tizen_fix

Change-Id: Ic03230504c400036acb480d22dacd59ed8a963cf