Add tile masking for coarse offsets computation

[ganctom]

Precision in estimating coarse offsets for em-datasets can often be improved with custom boolean masks for desired tile-pairs. This is particularly true for tile pairs at the edge or near region of interest, where charging and spatial deformations frequently occur. Below, the image on the left illustrates a stitched tile pair after coarse offset estimation done without a custom mask. Conversely, the image on the right demonstrates a stitched tile pair using a coarse offset estimated with the mask shown at the bottom of the middle row.

Updated code follows similar approach as in warp.py module, where tile_masks are also an option in render_tiles function.

[google-cla]

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