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/blenlib/BLI_kdopbvh.h

@@ -107,6 +107,7 @@ typedef void (*BVHTree_RangeQuery)(void *userdata, int index, const float co[3],
 typedef void (*BVHTree_NearestProjectedCallback)(
         void *userdata, int index,
         const struct DistProjectedAABBPrecalc *precalc,
+        const float (*clip_plane)[4], const int clip_plane_len,
         BVHTreeNearest *nearest);
 
 

source/blender/blenlib/intern/BLI_kdopbvh.c

@@ -2040,7 +2040,10 @@ static void bvhtree_nearest_projected_dfs_recursive(
 {
 	if (node->totnode == 0) {
 		if (data->callback) {
-			data->callback(data->userdata, node->index, &data->precalc, &data->nearest);
+			data->callback(
+			        data->userdata, node->index, &data->precalc,
+			        NULL, 0,
+			        &data->nearest);
 		}
 		else {
 			data->nearest.index = node->index;
@@ -2089,7 +2092,10 @@ static void bvhtree_nearest_projected_with_clipplane_test_dfs_recursive(
 {
 	if (node->totnode == 0) {
 		if (data->callback) {
-			data->callback(data->userdata, node->index, &data->precalc, &data->nearest);
+			data->callback(
+			        data->userdata, node->index, &data->precalc,
+			        data->clip_plane, data->clip_plane_len,
+			        &data->nearest);
 		}
 		else {
 			data->nearest.index = node->index;

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,