Tests pass so we are... almost cooking

Instead of using ChildOf, which has Transform implications, we created
our own relationship:

    ShapeBlocks(Vec<Entity>) <-> BlockOf { parent: Entity }

This relationship just informs how to place blocks with a
RelativePosition near their GridPosition parents without having to deal
with the Transform of the parent.

DESIGN.md

@@ -0,0 +1,45 @@
+# Design
+
+## Tetris
+
+Matrix Multiplication for rotating pieces.
+
+Each piece (shape) contains a Mat4 containing a representation of it's shape.
+For example:
+
+```
+0 1 0 0
+0 1 0 0
+0 1 0 0
+0 1 0 0
+```
+
+This is the classic `line` piece.
+
+And here it is on it's `up` side
+
+```
+0 0 0 0
+1 1 1 1
+0 0 0 0
+0 0 0 0
+```
+
+And here is the `t` piece
+
+```
+0 1 0
+1 1 1
+0 0 0
+```
+
+A matrix multiplication is applied to this Mat6 to achieve a piece rotation.
+
+When that matrix is updated, the 4 blocks parented to the shape are moved to reflect this new shape.
+
+This matrix also allows us to do checks to see if any of the blocks in the shape would intersect with another piece on the board.
+We can also check if a piece would go out of bounds during a move or rotation.
+
+We can use this to "plan -> validate -> commit" changes based on user input.
+
+Question: How the fuck do matrix multiplications work??