Fix bug #5870701 Thai text can be clipped when there are more glyphs generated...

Fix bug #5870701 Thai text can be clipped when there are more glyphs generated than the initial number of code points

- compute total advances correctly by iterating on the advances array for up to mShaperItem.num_glyphs
- update unit tests

Change-Id: I00af68bef88702215e9222ed80dbffcc81df51a7