Snap system: Adds support to Clip Planes and uses a clip plane to simulate occlusion

This patch adds support for clip_planes (ie ignore what is behind a face)...

The idea is to first execute a raycast to get the polygon to which the mouse cursor points.
Then a snap test is done on the vertices or edges of the polygon.
Then with the normal and location obtained in raycast a new clip_plane is created and the snap over the whole scene is processed ignoring the elements behind the clip_plane.

Here 2 gif of how the previous patch would work on blender2.79:

{F497176}

{F497177}

Reviewers: mont29, campbellbarton

Reviewed By: campbellbarton

Subscribers: bliblubli

Tags: #bf_blender_2.8

Differential Revision: https://developer.blender.org/D2527

source/blender/editors/transform/transform_snap.c

@@ -1201,6 +1201,7 @@ bool snapObjectsTransform(
 	        &(const struct SnapObjectParams){
 	            .snap_select = t->tsnap.modeSelect,
 	            .use_object_edit_cage = (t->flag & T_EDIT) != 0,
+	            .use_occlusion_test = t->scene->toolsettings->snap_mode != SCE_SNAP_MODE_FACE,
 	        },
 	        mval, dist_px, NULL,
 	        r_loc, r_no, NULL,