Added a few new things using block states.

* `//set ##*tag` sets all states in the tag (not just default state per type)
* `//set ^type` is a pattern changing block type but copying all valid existing states
* `//set ^[prop=val,...]` sets the property `prop` to `val` wherever the existing block has that property
* `//set ^type[prop=val,...]` does both of the above
Those work anywhere a pattern is taken, of course.

* The mask syntax `^[prop=val]` matches blocks with the property `prop` set to `val`, or blocks that don't have the property at all.
* The mask syntax `^=[prop=val]` only matches blocks that have the property.
Those work anywhere a mask is taken, of course. (`//mask`, `//gmask`, `//replace`, etc)

The `//drain` command now takes `-w` flag that removes the waterlogged state from blocks (in addition to removing water, as before).

worldedit-core/src/main/java/com/sk89q/worldedit/extent/buffer/ExtentBuffer.java

@@ -0,0 +1,97 @@
+/*
+ * WorldEdit, a Minecraft world manipulation toolkit
+ * Copyright (C) sk89q <http://www.sk89q.com>
+ * Copyright (C) WorldEdit team and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.sk89q.worldedit.extent.buffer;
+
+import com.google.common.collect.Maps;
+import com.sk89q.worldedit.WorldEditException;
+import com.sk89q.worldedit.extent.AbstractDelegateExtent;
+import com.sk89q.worldedit.extent.Extent;
+import com.sk89q.worldedit.function.mask.Mask;
+import com.sk89q.worldedit.function.mask.Masks;
+import com.sk89q.worldedit.math.BlockVector3;
+import com.sk89q.worldedit.world.block.BaseBlock;
+import com.sk89q.worldedit.world.block.BlockState;
+import com.sk89q.worldedit.world.block.BlockStateHolder;
+
+import java.util.Map;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+/**
+ * Buffers changes to an {@link Extent} and allows retrieval of the changed blocks,
+ * without modifying the underlying extent.
+ */
+public class ExtentBuffer extends AbstractDelegateExtent {
+
+    private final Map<BlockVector3, BaseBlock> buffer = Maps.newHashMap();
+    private final Mask mask;
+
+    /**
+     * Create a new extent buffer that will buffer every change.
+     *
+     * @param delegate the delegate extent for {@link Extent#getBlock(BlockVector3)}, etc. calls
+     */
+    public ExtentBuffer(Extent delegate) {
+        this(delegate, Masks.alwaysTrue());
+    }
+
+    /**
+     * Create a new extent buffer that will buffer changes that meet the criteria
+     * of the given mask.
+     *
+     * @param delegate the delegate extent for {@link Extent#getBlock(BlockVector3)}, etc. calls
+     * @param mask the mask
+     */
+    public ExtentBuffer(Extent delegate, Mask mask) {
+        super(delegate);
+        checkNotNull(delegate);
+        checkNotNull(mask);
+        this.mask = mask;
+    }
+
+    @Override
+    public BlockState getBlock(BlockVector3 position) {
+        if (mask.test(position)) {
+            return getOrDefault(position).toImmutableState();
+        }
+        return super.getBlock(position);
+    }
+
+    @Override
+    public BaseBlock getFullBlock(BlockVector3 position) {
+        if (mask.test(position)) {
+            return getOrDefault(position);
+        }
+        return super.getFullBlock(position);
+    }
+
+    private BaseBlock getOrDefault(BlockVector3 position) {
+        return buffer.computeIfAbsent(position, (pos -> getExtent().getFullBlock(pos)));
+    }
+
+    @Override
+    public <T extends BlockStateHolder<T>> boolean setBlock(BlockVector3 location, T block) throws WorldEditException {
+        if (mask.test(location)) {
+            buffer.put(location, block.toBaseBlock());
+            return true;
+        }
+        return false;
+    }
+}