Improve dist_***_to_corner_v3v3v3 precision

Remove offset before calculating distance.

Define 'plane3' to BLI_math, since we often don't need the 4th component.

source/blender/blenlib/BLI_math.h

@@ -43,6 +43,8 @@
  *  - ``m4`` = mat4 = matrix 4x4
  *  - ``eul`` = euler rotation
  *  - ``eulO`` = euler with order
+ *  - ``plane`` = plane 4, (vec3, distance)
+ *  - ``plane3`` = plane 3 (same as a ``plane`` with a zero 4th component)
  *
  * \subsection mathabbrev_all Function Type Abbreviations
  *