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Fix T66256: Context overrides crash when operators change context

Using context overrides in Python caused problems for any operator that
changed the context and require these changes to be read back.

CTX_wm_area_set() for e.g. would set the struct member but future
calls to CTX_wm_area() would still return the value defined by Python
callers context overrides.

This also resolves a mismatch between polling and calling operators
from Python, where poll would override the Python context where calling
only overrode the context when a new context was passed in.