CI(screendump): Support iterative filtering for screendump comparison

CI(screendump): Support iterative filtering for screendump comparison

Before two screendumps are compared for equality by calling
"VerifyScreenDump()", parts of their contents can be omitted
from comparison by executing arbitrary Vim commands written
in a filter file that shares its basename with screendumps.
Sometimes, such filtering can only be too general, as more
context is required in order to decide what parts to touch.
Two new arbitrary functions are therefore hooked in the body
of "VerifyScreenDump()" for the purpose of probing into the
current context and applying iterative filtering as needed.
A paired-up public implementation of each function is also
provided to expedite a workaround for #16559:
------------------------------------------------------------
source util/screendump.vim
let opts = {
    \ 'FileComparisonPreAction':
	\ function('g:ScreenDumpDiscardFFFDChars'),
    \ 'NonEqualLineComparisonPostAction':
	\ function('g:ScreenDumpLookForFFFDChars'),
\ }
call g:VerifyScreenDump(buf, basename, opts)
------------------------------------------------------------

related: #17704

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>