Improve Border and EmptyCell Graphics

[np-eazy]

• Indicator for off-screen blocks, cast dropzone whenever off-screen
• Indicator for direction of gravity and/or solid walls for each turn
• Make sure meter in emptyCell never exceeds [0, 1) range
• Check that things are being rendered in the right order so overlaps don't look weird

[np-eazy]

This can be achieved as follows:

• Define a number for BOARD_MARGIN along with all mentions of BOARD_SIZE. Introduce GLOBAL_BOARD_SIZE
• Render only the subarray as specified by BOARD_MARGIN
• For cells that spawn within the margin, have a DEAD_CELL enum at -1 which nops on each of its method calls.
• Whenever the board is rendered, it only renders the BOARD_SIZE square subarray at the center of the actual board.
• This also means the boundary coordinates will not be on the global boundaries and must be shifted accordingly.
• Refactor BoundarySets creation into an exported function that returns its originally single attribute. The function will have a margin number as an argument.
• Create a new class BoundaryPiece to put in BoundarySets rather than just 2d indices; this class will wrap around the corresponding boundary coordinate in the global size, and also have a flag & update() function for how present they are. BoundaryPieces also indicate which boundaries are solid, and experiment with different color schemes that work best design-wise.
• Add a max function on emptyCell's meter after all update. And for any cells which have things that might render outside; once this is figured out, it doesn't matter what order things are being rendered, no overlaps will form.

[np-eazy]

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