wineopenxr: Account for QPC to CLOCK_MONOTONIC offset in time conversion

Follow-up to #9347. Addresses the QPC to CLOCK_MONOTONIC clock mismatch by computing the offset at each conversion call as suggested by gofman. - Adds `get_qpc_to_monotonic_offset()` helper that calls `NtQueryPerformanceCounter` and `clock_gettime(CLOCK_MONOTONIC)` to compute the current offset - Applies offset in both `wine_xrConvertWin32PerformanceCounterToTimeKHR` and `wine_xrConvertTimeToWin32PerformanceCounterKHR` - Offset is computed per-call since it may drift over time Built and tested, works as expected. @gofman Ready for review when you have time.