Fix bi-directional text advances error

This bug occurs when bi-directional text is split up into left to right
(LTR) and right to left (RTL) runs. computeRunValues is done for each
run and the previous advance values are overwritten due to an offset
error. This change make sure that all new advance values are written at
the end.

Change-Id: I22cc0ae43ea9fe7862dc58ec33d7bcf8597ccbac