Fix rendering performance issue when using :version or echo commands

This fixes 10.14 Mojave's CoreText renderer taking a long time to render
:version / :ls / :! / :echo or similar commands.

This issue happened because the way Vim echos the output of those
commands is by issuing a draw calls in the pattern of "delete 1 line,
draw some text, delete another line...". Each line delete causes the
renderer to do a scroll. The pre-Mojave renderer relies on calling
scrollRect: but this doesn't work in Mojave anymore since that function
is deprecated and doesn't work in layer-backed views (which are now
mandatory). The new renderer's scroll implementation is a lot slower
since it's doing image blits on CPU.

The fix is to implement a draw command optimizer that pre-processes the
draw calls first. It works by batching together all the "delete 1 line"
calls and combine into a single "delete N lines" call and put that in
the beginning, and fixing up all the other draw string command so they
draw to the right line instead of needing to be scrolled up. This makes
:version or the other calls feel instaneous now.

This fix is ultimately a hack and an intermediary solution before the
renderer can be replaced (since the slow CPU scrolling causes normal
usage to feel sluggish as well) by a GPU-based renderer and/or a
glyph-based one that caches the state of the texts so repeated scrolling
can be done by shuffling the glpyh data around instead of an actualy image
blit.

Fix #815

Author: ychin