canvas: Change default transform to remove z

[johannes-wolf]

Change the default transformation matrix to have z coordinates of zero after transformation.
This is actually a fix, as the final 2D projection should not have a z-value other than zero.

With this change, anchors in 3D space become 2D and thus elements like rect, Bézier or Catmull, … will correctly work when using them with anchors to 3D elements. Intersection between 3D elements will also work (in 2D space).

See: #502 and #493.

[johannes-wolf]

I guess hand writing the matrix, instead of using 2 multiplications, is more clear.

[johannes-wolf]

With this PR you can get TikZ like marks on 3D lines:

  set-style(mark: (end: ">", transform-shape: false))
  line((0,0), (1,0))
  line((0,0), (0,1))
  line((0,0), (0,0,1))

[fenjalien]

• Add a comment to the default transform matrix to say what transforms it has e.g. shear then scale... xyz
• Check if the manual says anything about the default transform, update it if so
• Update changelog

[johannes-wolf]

• Add a comment to the default transform matrix to say what transforms it has e.g. shear then scale... xyz
• Check if the manual says anything about the default transform, update it if so
• Update changelog

Added a line to the changelog and documentation about the default matrix.