Modifiers: Add wrapper functions with Mesh / DerivedMesh conversion

Makes the follow changes:

- Add new `deform*` and `apply*` function pointers to `ModifierTypeInfo` that take `Mesh`, and rename the old functions to indicate that they take `DerivedMesh`. These new functions are currently set to `NULL` for all modifiers.
- Add wrapper `modifier_deform*` and `modifier_apply*` functions in two variants: one that works with `Mesh` and the other which works with `DerivedMesh` that is named with `*_DM_depercated`. These functions check which type of data the modifier supports and converts if necessary
- Update the rest of Blender to be aware and make use of these new functions

The goal of these changes is to make it possible to port to using `Mesh` incrementally without ever needing to enter into a state where modifiers don't work. After everything has been ported over the old functions and wrappers could be removed.

Reviewers: campbellbarton, sergey, mont29

Subscribers: sybren

Tags: #bf_blender_2.8

Differential Revision: https://developer.blender.org/D3155

source/blender/modifiers/intern/MOD_lattice.c

@@ -137,12 +137,21 @@ ModifierTypeInfo modifierType_Lattice = {
 	                        eModifierTypeFlag_AcceptsLattice |
 	                        eModifierTypeFlag_SupportsEditmode,
 	/* copyData */          copyData,
-	/* deformVerts */       deformVerts,
+
+	/* deformVerts_DM */    deformVerts,
+	/* deformMatrices_DM */ NULL,
+	/* deformVertsEM_DM */  deformVertsEM,
+	/* deformMatricesEM_DM*/NULL,
+	/* applyModifier_DM */  NULL,
+	/* applyModifierEM_DM */NULL,
+
+	/* deformVerts */       NULL,
 	/* deformMatrices */    NULL,
-	/* deformVertsEM */     deformVertsEM,
+	/* deformVertsEM */     NULL,
 	/* deformMatricesEM */  NULL,
 	/* applyModifier */     NULL,
 	/* applyModifierEM */   NULL,
+
 	/* initData */          initData,
 	/* requiredDataMask */  requiredDataMask,
 	/* freeData */          NULL,