feat(PlanarFreeHandRoiTool) : implementing PlanarFreeHandRoiTool on oblique plane

[Celian-abd]

Context

Right now, if we try to draw an annotation using the PlanarFreeHandRoiTool on a non orthogonal plane, it will show the following error : 'No support yet for oblique plane planar contours'.

Changes & Results

• Changed getSubPixelSpacingAndXYDirections.ts to add the oblique case.

Testing

• Use the crosshair to rotate
• Draw an annotation on a non orthogonal viewport

Checklist

PR

• [] My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• [] My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• [] The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• [] "OS:
• [] "Node version:
• [] "Browser:

[sedghi]

Thank you for this groundwork on removing the oblique-plane restriction; #2744 builds directly on this implementation and additionally corrects the stats sampling for oblique MPR viewports, so we are consolidating review there. Credit for the approach originates here.