- ghost fcurve drawing uses Show Extrapolation setting

Graph Editor: FCurve Extrapolation Visibility T76472

Adds toggle to graph editor (View->Show Extrapolation). When disabled, then fcurves only draw over the keyframe range. For baked fcurves, the range is all sampled points. For ghost fcurves, extraplation is always drawn since the sampled points may include extrapolated points of the original fcurve.

Show Extrapolation is the default.
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**Technical Notes**:
- `draw_fcurve_curve()`: There are now explicit checks for floating point poor accuracy cases. Without them, then there would be visual flickering when no extrapolation is drawn and there is a discontinuity at the end (due to cyclic modifier). This removes the need for the `+samplefreq` in `etime = v2d->curxmax + samplefreq`. It also removes the need to do so on `fcu_start/fcu_end`.
- `draw_fcurve_curve()`: Besides the previous note, the only other meaningful change is the `if(!draw_extrapolation)` branch which just changes `stime/etime` to match the keyframe bounds instead.

Differential Revision: https://developer.blender.org/D10442