… the same

Note: cimgui is 1.92.3 due to cimgui/cimgui#307

- 8a1b4c3 for IM_ASSERT changes
- 3d5246f "macos: Enable default clipboard implementation in imconfig.h"

- This fixes test_simple.py:test_style_access test that was failing due to mismatched
  enum values.  Unsurprisingly using cimgui definitions from another version
  didn't quite work because new enum values were added for 'imgui.Col'

  Style access was probably kinda working, but the test was written
  with the assumption that the enum values are in a contiguous range
  as it loops over the color slots from zero to max Col slot.

- 8a1b4c3 for IM_ASSERT changes
- 3d5246f "macos: Enable default clipboard implementation in imconfig.h"

…f object lifetimes

This is needed for DrawList.add_callback calls which must be designed so that
the Python callables are kept alive via object references and don't get
deallocated before the next `imgui.new_frame()` is called.

The DrawList getters for the Context were changed from `nb::rv_policy::reference_internal`
to `nb::rv_policy::reference` to make the refcounts testing make more sense.  Sure
it's not safe if you hold on to a drawlist handle until after you've destroyed the ImGui context
and then try to call its methods, but that's not a very sensible thing to do.

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