Improve char mirroring in TextLayoutCache

- now use ICU u_isMirrored() instead of a small hardcoded list of unicode points

see bug #5961254 Harfbuzz should be able to support Bidi_mirrored unicode attribute

Change-Id: I3243a58558a97930f0e7fdf5e9c1d5695d9393de
1 file changed