Incorrect geometry for OnMouseMove event on Slider with RightToLeft flow direction

When a slider is in RTL flow direction, and thus its position is changed, the handle is not drawn at the correct position during mouse drag.

Instead, the handle draws as if the slider was not moved by the RTL flow direction.

Repro from a blank project:

1. New Widget Blueprint. Build this tree, leaving every Flow Direction Preference at Inherit except where noted:
   SizeBox Flow Direction Preference = Right To Left
   HorizontalBox
   SizeBox Width Override 400, Height Override 40
   Slider Orientation Horizontal, Value 0.5
   Spacer Size 400 x 40
   The preference has to sit above the HorizontalBox rather than on it, so the box inherits the direction.
   The Spacer has to be a different width from the Slider, or mirroring moves the Slider 0px and nothing shows.
2. Compile and save, then show the widget: parent it to EditorUtilityWidget and use Run Editor Utility Widget,
   or Create Widget + Add to Viewport in PIE.
3. Check the Slider painted in the right half of the row. If it sits at the left edge the flow direction is not
   reaching the box and nothing below will reproduce.
4. Keeping the button held, drag left and right.

Observe: the handle jumps to the far right on the first move.
Expected: the handle tracks the cursor, and because the flow is mirrored, dragging right lowers the value.