Refactor: Simplify mesh edit mode modifier evaluation

Instead of keeping track of a local array of positions in the modifier
stack itself, use the existing edit mode SoA "edit cache" which already
contains a contiguous array of positions. Combined with positions as a
generic attribute, this means the state is contained just in the mesh
(and the geometry set) making the code much easier to follow.

To do this we make more use of the mesh wrapper system, where we can
pass a `Mesh` that's actually stored with a `BMesh` and the extra
cached array of positions. This also resolves some confusion-- it was
weird to have the mesh wrapper system for this purpose but not use it.

Since we always created a wrapped mesh in edit mode, there's no need
for `MOD_deform_mesh_eval_get` at all anymore. That function was quite
confusing with "eval" in its name when it really retrieved the original
mesh.

Many deform modifiers had placeholder edit mode evaluation functions.
Since these didn't do anything and since the priority is node-based
deformation now, I removed these. The case is documented more in the
modifier type struct callbacks.

It's worth landing this early in Bcon1 because there are probably some
edge cases I'm missing and this area isn't covered well by tests. But I
think the modifier stack simplification is absolutely worth it, and
opens other cleanup opportunities as well.

@@ -1212,3 +1201,2 @@
     const ModifierTypeInfo *mti = BKE_modifier_get_info((ModifierType)md->type);
-
-    if (!editbmesh_modifier_is_enabled(scene, ob, md, mesh_final != nullptr)) {
+    if (!editbmesh_modifier_is_enabled(scene, ob, md, i == 0)) {

@@ -171,0 +169,4 @@
+{
+  switch (mesh->runtime->wrapper_type) {
+    case ME_WRAPPER_TYPE_BMESH:
+      if (mesh->runtime->edit_data->vertexCos) {

@@ -17,3 +17,3 @@
 typedef struct EditMeshData {
   /** when set, \a vertexNos, polyNos are lazy initialized */
-  const float (*vertexCos)[3];
+  float (*vertexCos)[3];

@@ -179,0 +188,4 @@
+      if (mesh->runtime->edit_data->vertexCos == nullptr) {
+        mesh->runtime->edit_data->vertexCos = editbmesh_vert_coords_alloc(mesh->edit_mesh);
+      }
+      return mesh->runtime->edit_data->vertexCos;