Description:
------------
Since we have ob_get_level() we kind of miss ability to reference that stack from 
nested active stacks. In recursive content generators this will avoid the need to 
loop through the final output to determine the bounds of nested stacks and do 
operations with that(those) segment(s), such as replacement and so on.

Test script:
---------------
ob_start();
echo "111"; // <-- this is the stack of interest
ob_start();
echo "22";
ob_start();
echo "3";
$cur_stack_len = ob_get_length(); // length of current stack (level 3) is 1
$top_stack_len = get_length_of_ob_stack_by_level(1); // expected length here should be 3 (strlen("111") == 3)
ob_end_clean();
ob_end_clean();
echo "some more chars to change length of stack 1";
ob_end_clean();
echo $top_stack_len; // I'm expecting to see 3 here.

Expected result:
----------------
3

Actual result:
--------------
not implemented