Displace Modifier - Optimisation Tweaks for jpbouza and ZanQdo

- When strength is 0, there's no need to perform any of the
calculations at all
- When the vertexgroup weight for a vert is set to 0, skip evaluating
the modifier for that vertex as it should result in no-change to the
final result

source/blender/modifiers/intern/MOD_displace.c

@@ -260,6 +260,7 @@ static void displaceModifier_do(
 	float (*tex_co)[3];
 
 	if(!dmd->texture) return;
+	if(dmd->strength == 0.0f) return;
 
 	defgrp_index = defgroup_name_index(ob, dmd->defgrp_name);
 
@@ -284,7 +285,7 @@ static void displaceModifier_do(
 					break;
 				}
 			}
-			if(!def_weight) continue;
+			if(!def_weight || def_weight->weight==0.0f) continue;
 		}
 
 		texres.nor = NULL;