Gpencil: several weight paint additions

This patch adds several tools and options to the weight paint mode of
Grease Pencil.

More tools
- Blur tool: smooths out vertex weights, by calculating a gaussian blur
of adjacent vertices.
- Average tool: painting the average weight from all weights under the
brush.
- Smear tool: smudges weights by grabbing the weights under the brush
and 'dragging' them.

Draw tool: switch easily between 'add' and 'subtract'
With the + and - icons in the toolbar, the user can easily switch
between adding and subtracting weight while drawing weights.
With shortcut 'D' you can toggle between these two.

New option: auto-normalize
The auto-normalize options ensures that all bone-deforming vertex
groups add up to 1.0 while weight painting.

Radial control for weight
With Ctrl-F a radial control for weight is invoked (in addition
to the radial controls for brush size and strength).

Sample weight
With Ctrl-RMB the user can sample the weight. This sets the brush
Weight from the weight under the cursor.

Quick bone selection
When painting weights in vertex groups for bones, the user can
quickly switch to another vertex group by clicking on a bone with
Ctrl-LMB.
For this to work, follow these steps:
- Select the armature and switch to Pose Mode.
- Select your Grease Pencil object and switch immediately to
Weight Paint Mode.
- Select a bone in the armature with Ctrl-LMB. The corresponding
vertex group is automatically activated.

Technical note: where possible, the same logic and conventions
as in the 3D weight paint section of Blender were followed.

@@ -1417,0 +1418,4 @@
+        
+        gp_settings = brush.gpencil_settings
+        text = "" if compact else "Direction"
+        layout.prop(gp_settings, "direction", expand=True, text=text)

@@ -45,6 +45,7 @@ def generate_from_enum_ex(
                 dict(
                     idname=idname_prefix + name,
                     label=name,
+                    description=(enum.description),

@@ -4253,1 +4253,4 @@
 
+  if (!MAIN_VERSION_ATLEAST(bmain, 306, 5)) {
+    /* Rename Grease Pencil weight draw brush. */
+    do_versions_rename_id(bmain, ID_BR, "Draw Weight", "Weight Draw");

@@ -164,2 +235,2 @@
-    if (buffer_array != NULL) {
-      memset(buffer_array, 0, sizeof(tGP_Selected) * *buffer_size);
+/* Ensure a vertex group for the weight paint brush. */
+static void gpencil_vertex_group_ensure(tGP_BrushWeightpaintData *gso)

@@ -166,0 +277,4 @@
+/* Get boolean array of vertex groups deformed by GP armature modifiers.
+ * GP equivalent of `BKE_object_defgroup_validmap_get`.
+ */
+bool *gpencil_vgroup_bone_deformed_map_get(Object *ob, const int defbase_tot)

@@ -166,0 +324,4 @@
+
+  /* Mark vertex groups with reference in the bone hash table. */
+  vgroup_bone_deformed = MEM_mallocN(sizeof(*vgroup_bone_deformed) * defbase_tot,
+                                     "wpaint bone deformed map");

@@ -248,0 +632,4 @@
+
+  /* Calculate the average (=blurred) weight. */
+  float blur_weight = 0.0f, dist_sum = 0.0f;
+  int i, count = 0;

@@ -248,0 +684,4 @@
+  float point_dot[8] = {0};
+  float point_dir[2];
+  float score_max = 0.0f, dist_min = FLT_MAX, dist_max = 0.0f;
+  int i_max = -1, count = 0, i;

@@ -526,3 +1129,4 @@
    * required to do some step. Now is not used, but the operator is ready.
    *--------------------------------------------------------------------- */
+  int gps_index = -1;
   LISTBASE_FOREACH (bGPDstroke *, gps, &gpf->strokes) {

@@ -3095,2 +3091,2 @@
-     * are in object mode it might falsely toggle object selection. Alternatively, this could be
-     * put in the poll function instead. */
+  if (obact && obact->type == OB_GPENCIL_LEGACY &&
+      GPENCIL_NON_WEIGHT_MODE((bGPdata *)obact->data)) {

@@ -170,0 +409,4 @@
+    return true;
+  }
+
+  for (i = dvert->totweight, dw = dvert->dw; i != 0; i--, dw++) {

@@ -170,0 +434,4 @@
+  if (sum_lock >= 1.0f - VERTEX_WEIGHT_LOCK_EPSILON) {
+    /* locked groups make it impossible to fully normalize,
+     * zero out what we can and return false */
+    for (i = dvert->totweight, dw = dvert->dw; i != 0; i--, dw++) {

@@ -170,0 +448,4 @@
+  if (sum_unlock != 0.0f) {
+    fac = (1.0f - sum_lock) / sum_unlock;
+
+    for (i = dvert->totweight, dw = dvert->dw; i != 0; i--, dw++) {

@@ -170,0 +462,4 @@
+    fac = (1.0f - sum_lock) / tot;
+    CLAMP(fac, 0.0f, 1.0f);
+
+    for (i = dvert->totweight, dw = dvert->dw; i != 0; i--, dw++) {

@@ -911,0 +1642,4 @@
+
+      /* Look for two closest points. */
+      for (int i = 0; i < gps->totpoints; i++) {
+        bGPDspoint *pt = gps->points + i;

@@ -911,0 +1652,4 @@
+
+        if ((dist < closest_dist[0]) || (dist < closest_dist[1])) {
+          /* Get weight. */
+          MDeformVert *dvert = gps->dvert + i;