Fix: preserve original Figure DPI in pickle to prevent HiDPI doubling (macOSX)

[emerson-gray]

Problem

• On HiDPI backends (e.g., macOSX on Retina), FigureCanvasBase scales Figure.dpi by device pixel ratio (DPR) based on figure._original_dpi. On unpickle, FigureCanvasBase(self) sets figure._original_dpi from the current (possibly scaled) dpi. When the backend applies DPR again, dpi is multiplied again, doubling on each pickle/unpickle cycle.

Root cause

• Figure.getstate persists the runtime _dpi (which may be scaled), not the baseline, unscaled dpi. This breaks the invariant that _original_dpi should reflect the figure's baseline dpi before DPR.

Fix

• In Figure.getstate, persist the unscaled dpi by setting state["_dpi"] = getattr(self, "_original_dpi", self._dpi) before returning. This ensures unpickled figures start from baseline dpi, and DPR is applied exactly once.

Tests

• Add test_hidpi_pickle_unpickle_does_not_double_dpi to tests/test_pickle.py that emulates DPR=2 via fig.canvas._set_device_pixel_ratio(2) and verifies dpi remains stable (200) across repeated pickle/unpickle cycles.

Compatibility and risk

• Non-HiDPI backends (DPR=1) are unaffected because _original_dpi == _dpi.
• Older pickles created before this change will continue to load as before; they may still reflect the previously scaled dpi. The change only affects newly created pickles by making them future-proof against cumulative scaling.

Target

• Base branch: matplotlib__matplotlib-23476