Add bounds checking in WorldSlice

[embeddedt]

Currently, calling WorldSlice#getBlockState or related methods with a block position that wasn't cloned will either return null or throw ArrayIndexOutOfBoundsException. This tends to cause crashes in mods, and is not very useful for debugging.

To solve this, we check that the given position is within bounds, and return a sane default value if not:

• Blocks.AIR.getDefaultState() for block states
• 0 for light levels
• null for block entities and block entity render attachments

[jellysquid3]

Checking the section index against the bounds of the array will probably not do what you want. Each coordinate has to be checked for the out-of-range condition individually. For example, coordinates [4, 0, 0] will produce the same index as coordinates [1, 0, 1]. While that is still in bounds of the flat array, it will produce hard to debug issues that manifest as seemingly random corruption.

[jellysquid3]

Some other thoughts:

• We could just initialize the block data arrays to the default state, rather than leaving null entries in there. That would eliminate the need for checking if there is a null entry.
• The implementation could be changed to store a single 34x34x34 array of block data, rather than a table of sections -> block data arrays. That way, we can consolidate the "out of bounds" and "no data exists" cases into one, and simplify the code a bit. This might actually be faster, anyways, since it reduces the amount of indirection and index calculations.