Returns whether this are complete.
The timing information in a `GdkFrameTimings` is filled in incrementally as the frame as drawn and passed off to the window system for processing and display to the user. The accessor functions for `GdkFrameTimings` can return 0 to indicate an unavailable value for two reasons: either because the information is not yet available, or because it isn't available at all.
Once this function returns true for a frame, you can be certain that no further values will become available and be stored in the `GdkFrameTimings`.
| this |
a `GdkFrameTimings` |
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true if all information that will be available for the frame has been filled in. |