Vte is not sending AT-SPI correct delete events when less is open in a terminal emulator, and when the viewed file contains empty lines.

As a result, BRLTTY shows terminal contents incorrectly.

To reproduce:
1) Create a text file containing lots of empty lines (say 100)
2) Open this file with less
3) Press the down arrow key to scroll the file

After step 3, vte should send a delete event for the colon character which less displays on the bottom line of the terminal. But because vte misses this, BRLTTY's screen buffer gets messed up.

This is a problem, because the same behaviour can be triggered by scrolling text left (using the right arrow key) in less, and because this can indicate some deeper problem in vte's event sending logic.

When viewing a file containign empty lines in less, BRLTTY receives the following AT-SPI events:

# First less prints the empty lines from my test file. vte correctly sends events for this.
brltty: insert 8 from 20
brltty: '


empty'

brltty: caret move to 28
# The 'empty' above is the name of my empty file which less prints to the bottom line when it starts
# Now I started to press the down arrow key

# The last line of the terminal now reads: 'empty' in BRLTTY

brltty: delete 1 from 0
brltty: '
'
brltty: caret move to 24
brltty: delete 1 from 0
brltty: '
'
brltty: insert 1 from 23
brltty: ':'
# BRLTTY now thinks that the last line reads: 'em:pty'

brltty: delete 1 from 0
brltty: '
'
brltty: insert 1 from 23
brltty: ':'
# BRLTTY thinks that the last line now reads: 'em::pty'

brltty: delete 1 from 0
brltty: '
'
brltty: insert 1 from 23
brltty: ':'
# BRLTTY thinks that the last line now reads: 'em:::pty'
# and so forth