Use individual glyph positions to determine text bounds.

bug:8766924

Previously text bounds were calculated to be from 0 to totalAdvance in
the X, and from the font's top to bottom. These are incorrect,
especially in light of the font fallback mechanism.

Now, we calculate the bounds of the text as we layout each glyph.
Since these are much tighter bounds in the common case, this
significantly reduces the amount of clipping required (which in turn
enables more aggressive text merging).

Change-Id: I172e5466bf5975bf837af894a9964c41db538746