Fix gizmo clipping again

[atlv24]

Objective

• alternative to Fix problems displaying grid gizmos #21502
• also undoes Fix gizmo near plane clipping bug #20890
• does not regress Gizmos flip axes when values are large #19205
• fixes Gizmo lines culled incorrectly in 3D with depth_bias -1 #21489 and Gizmo Grid incorrect looking lines #21479

Solution

the fix i originally made was wrong. the reason it worked was because it failed the conditional and did not clip the lines in some cases where the original bug was exhibited. the condition was correct before, it turns out, but the clipping logic is wrong: an epsilon is added, with a comment saying it is to avoid falsely culling the vertex due to floating point imprecision leaving it just barely in front of the near plane. However, i do not see why an epsilon should be needed here, or why this should be avoided: triangle primitives are regularly intersecting the clipping plane, that is standard order of business. The entire primitive is not discarded if only part of it is clipped, instead only the visible pixels are rasterized.

Testing

• original bug test case by Antony looks right
• both grid test cases look right
• gizmos examples look right

[janis-bhm]

Checked this out and tested with 3d_gismos example.
I don't fully understand the clip space math going on here, but I believe it's mix(a, b, (distance from a to near plane over distance from a to b) and the signs are flipped for some kind of optimisation?