sub: do not copy the target image if there is no OSD/subs

It's not easy to tell whether the OSD/subs are empty, or if something is
drawn. In general you have to use osd_draw() with a custom callback. If
nothing is visible, the callback is never invoked. (The actual reason
why this is so "hard" is the implementation of osd_libass.c, which
doesn't allow separating rendering and drawing of OSD elements, because
all OSD elements share the same ASS_Renderer.)

To simplify avoiding copies, make osd_draw_on_image() instead of the
caller use mp_image_make_writeable(). Introduce osd_draw_on_image_p(),
which works like osd_draw_on_image(), but gets the new image allocation
from an image pool. This is supposed to be an optimization, because it
reduces the frequency of large allocations/deallocations for image data.

The result of this is that the frequency of copies needed in conjunction
with vf_sub, screenshots, and vo_lavc (encoding) should be reduced.
vf_sub now always does true pass-through if no subs are shown.

Drop the pts check from vf_sub. This didn't make much sense.

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