Added a SolidBlockMask (#solid).

src/main/java/com/sk89q/worldedit/masks/SolidBlockMask.java

@@ -0,0 +1,15 @@
+package com.sk89q.worldedit.masks;
+
+import com.sk89q.worldedit.EditSession;
+import com.sk89q.worldedit.Vector;
+import com.sk89q.worldedit.blocks.BlockType;
+
+/**
+ * Works like {@link ExistingBlockMask}, except also dealing with non-solid non-air blocks the same way as with air.
+ */
+public class SolidBlockMask extends AbstractMask {
+    @Override
+    public boolean matches(EditSession editSession, Vector pos) {
+        return !BlockType.canPassThrough(editSession.getBlockType(pos), editSession.getBlockData(pos));
+    }
+}