Fix RTL reading glyphs out of buffer. 1. LayoutText was not using reversed indexes. 2. hb_set_length() sets from beginning.
Fix RTL reading glyphs out of buffer.
1. LayoutText was not using reversed indexes.
2. hb_set_length() sets from beginning.

Resolve both issues by just hb_buffer_reverse() and no more reversed indexes in
UpdateBuffer.

PiperOrigin-RevId: 155818962
Change-Id: I4197f3a7c5a10886a809a2a7d2dfd7969e19548f

google/flatui,google/flatui,google/flatui,google/flatui