Missed an assignment for end_run when the end cursor is provided and the
end position doesn't cause a split.

The test that succeeded from this change was as a result of allowing the
end of the character format change be specified using NULL as the rest
of the text.  Before, the end paragraph run at the end of the text was
not being set for this case, when all the text was supposed to have its
character format changed.

Prevent extra conversions from character offset to ME_Cursor.

Plenty of places in the code find following or preceding runs, then
afterwards find the paragraph from the run.  This is inefficient because
the same linked list is used for both runs and paragraphs, so changes in
paragraphs can be detected while returning the next or previous run.

The code was previously getting the same text in the loop from the
editor, and it was converting each of the prefixes to compare against
for each URL candidate.

Before a single length was used for the number of characters to retrieve
from the text, and to keep track of the size of the buffer.  These are
not equivalent, since there is a possible end of line conversion.

Previously the only convenient way to get the start and end of the
selection was through offsets, which eventually need to get converted
back into items in the linked list storing the text.  The new function
will help with eliminating these inefficiencies.

This function will make it easier to work with ME_Cursor objects, which
should be used in a lot of places instead of character offsets (which
often require seeking through the linked lists to perform operations
with).