evas_textblock: Make maxascent/maxdescent variables local.
evas_textblock: Make maxascent/maxdescent variables local.

They are used in single function, so don't keep them in structure.
As a side-effect rework _layout_line_advance for better readability.

git-svn-id: 6d771e449150288cc513807b7f4d2af31e9482bd@46872 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33

TizenChameleon/evas,TizenChameleon/uifw-evas,nashidau/Evas-Next,TizenChameleon/uifw-evas,TizenChameleon/evas,TizenChameleon/evas,TizenChameleon/evas,nashidau/Evas-Next,nashidau/Evas-Next,TizenChameleon/uifw-evas,TizenChameleon/uifw-evas