[cairo] Text polish and cleanup

Panic when hit_test_position passed non char boundary

This matches the behaviour on the other platforms.

Remove a bunch of old cairo text tests

These were always fragile, and are now replaced by the shared tests
in piet-common.

Fixup hit-testing

- Keep track of the line x_offsets, to correctly resolve positions
  within lines
- For hit_test_position, don't use text.len()

Fixup hit_test_position again

Fixup trailing whitespace and ink rect calculations

Update snapshots

Increase fudge factor in tests to accomodate pango

Last bit of pre-merge cleanup