This makes it possible to do a border render inside a viewport even
when not looking through the camera.
Render border could be defined by Ctrl-B shortcut (works for both
camera render border and viewport render border).
Camera render border could still be defined using Shift-B (so no
muscule memory would be broken). Currently used a special flag of
operator to do this, otherwise you'll need to either two operators
with different poll callback or it could go into conflict with a
border zoom,
Border render of a viewport could be enabled/disabled in View
panel using "Render Border" option.
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