Implement AtkText::get_character_extents and AtkText::get_range_extents

Bug: 934274
Change-Id: I53c9545ed4f4cc91b11978a3b71012a3d5d1a834
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1520504
Commit-Queue: Martin Robinson <mrobinson@igalia.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642471}
10 files changed