Implement --input-tileset (#1464)

As discussed in https://github.com/gbdev/rgbds/issues/575#issuecomment-1991456862

test/gfx/input_tileset.flags

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+-i input_tileset.in.2bpp
+-u

test/gfx/input_tileset_deduped.err

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+error: The input tileset's tile #7 was deduplicated; please check that your deduplication flags (`-u`, `-m`) are consistent with what was used to generate the input tileset
+Conversion aborted after 1 error

test/gfx/input_tileset_deduped.flags

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+-i input_tileset_deduped.in.2bpp
+-u
+-o result.2bpp

test/gfx/input_tileset_extra.err

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+error: Tile at (0, 0) is not within the input tileset, and `-o` was not given!
+error: Tile at (0, 8) is not within the input tileset, and `-o` was not given!
+error: Tile at (8, 8) is not within the input tileset, and `-o` was not given!
+Conversion aborted after 3 errors

test/gfx/input_tileset_extra.flags

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+-i input_tileset.in.2bpp
+-u