evas font: do floating point division for calculating more accurately

evas font: do floating point division for calculating more accurately

Summary:
Assigning a result of integral division to a double type variable is
not useful for next division calculation. For more accurate calculation,
it needs to be casted to double before doing division.
It does not fix some bugs. It was reported by a code quality advisor.

Test Plan: N/A

Reviewers: raster, cedric, jpeg, herdsman, eunue

Reviewed By: cedric

Differential Revision: https://phab.enlightenment.org/D5069

Signed-off-by: Cedric Bail <cedric@osg.samsung.com>