[Feature] Distance Measurement Tool

[brenocq]

Feature description
Users may need to verify dimensions or analyze the spatial relationship between specific areas of a plot. This feature proposes a Measurement Tool that allows users to determine the precise distance between two points on the plot's bounding box planes.

Workflow:

1. Activation: The user activates the tool via the Plot Context Menu (e.g., Right-click -> "Measure").
2. Selection: The user clicks a point on any visible plane (grid) of the plot box to set Point A.
3. Measurement: The user clicks a second point on any visible plane to set Point B.
4. Feedback:
   • Markers appear at both points.
   • A line connects Point A and Point B.
   • A tooltip or overlay displays the $\Delta X$, $\Delta Y$, $\Delta Z$, and the Euclidean distance (Length) between the points.
5. Cancellation/Reset: Pressing ESC or right-clicking cancels the operation or clears the current measurement.

Constraints:

• Initially, point selection is restricted to the plot box planes (XY, XZ, YZ faces).
• The tool calculates distances based on the plot coordinates, not screen pixels.

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Tasks

• Internal State (implot3d_internal.h)
  • Add state variables to ImPlot3DPlot to track the measurement status:
    • MeasureMode (Enum: None, SelectingP1, SelectingP2, Finished).
    • MeasureP1 and MeasureP2 (ImPlot3DPoint).
• User Interface (implot3d.cpp)
  • Add a "Measure" menu item to ShowPlotContextMenu to toggle the mode.
• Input Handling (implot3d.cpp)
  • In HandleInput, implement logic to detect mouse clicks when MeasureMode is active.
  • Utilize GetMouseOverPlane and PixelsToPlotPlane to calculate the 3D coordinate of the mouse cursor projected onto the hovered plot face.
  • Update MeasureP1 and MeasureP2 on click events.
• Rendering (implot3d.cpp)
  • Render circles at MeasureP1 (and MeasureP2 if set).
  • Render a line connecting the two points.
  • Render a text tooltip or overlay box displaying:
    • dX: ...
    • dY: ...
    • dZ: ...
    • Len: ...
• Demo (implot3d_demo.cpp)
  • Add a demonstration of the tool in the implot3d_demo.cpp (can be implicitly tested via the context menu on existing plots).