[MCM-1493] Loading small dripleaf blocks through structures creates overlapping blocks

Bug has been found in all 1.18 builds, yet does not occur in the latest 1.17 build.

When small dripleaf blocks are loaded as part of a structure, any blocks that should be overwritten by the small dripleaf blocks, now become an "Extra Block" within the small dripleaf (similar to how flowers can exist within top snow layer). The overwritten blocks are still visually there, but do not have a collision box until the small dripleaf blocks are broken, in which the original blocks will have their collision box and functionality again.

This bug impacts one of our planned major releases that heavily features the new Caves and Cliffs blocks. Everything was working perfectly in 1.17, yet now there are these "ghost blocks" covering all the small dripleaves that have been loaded via structures.

Repro Steps
- Step 1: Download the attached world and open it with Minecraft in any 1.18 build.
- Step 2: Once in the world, notice there is structure A with several blocks and structure B with small dripleaf blocks.
- Step 3: Now notice the platform with the command blocks. Click the button on the LEFT command block to load structure A.
- Step 4: With structure A now loaded, click the button on the RIGHT command block to load structure B.
- Step 5: Notice both structures exist in the same space.
- Step 6: Notice you can walk through the blocks on the platform.
- (Optional): You can repeat the test with any blocks in the 2x2x2 space above the moss platform, not just ones from structure A. (as shown in the video below)
Repro Rate

Once completed the above repro steps, notice the following:
Notice both structures now exist within the same space, as the original blocks have become an "Extra Block" of the small dripleaves.
Notice you can walk through the blocks on the platform, with many of the "ghost" blocks obscuring your view when inside them.
Notice that if you destroy the upper block of the small dripleaf blocks, the original blocks will still exist with their collision box and functionality again.
Notice if you destroy the lower block of the small dripleaf blocks, the original block that also exists within the lower block will disappear, while the original upper block will still exist with their collision box and functionality again.

When loading a structure with small dripleaf blocks, the small dripleaf blocks should overwrite blocks as they did in 1.17

Inspecting the merged small dripleaf blocks with the block debug ImGui shows that the overwritten blocks now exist within the "Extra Block" tag