Avoid flipping normal based on facing direction when calculating SDF

[clayjohn]

Fixes: #101746
Regression from: #100441

This reverts us back to the state prior to #100441. By switching to geo_normal we were implicitly adding the following lines of code which flip the normal based on the direction that the surface is viewed from:

godot/servers/rendering/renderer_rd/shaders/forward_clustered/scene_forward_clustered.glsl

Lines 1171 to 1177 in 6dc78c8

	vec3 normal = normal_interp;
	#if defined(DO_SIDE_CHECK)
	if (!gl_FrontFacing) {
	normal = -normal;
	}
	#endif // DO_SIDE_CHECK
	#endif // NORMAL_USED

Normally this code is desirable as it allows users to get acceptable normals when viewing backfaces. However, for rendering the SDF, we are viewing the surface from multiple sides and we do not want the normal to flip based on view direction.

[stuartcarnie]

LGTM!

[Repiteo]

Thanks!