TileLayoutStitcher.stitch sparseness assumption

[moradology]

The current stitch function is a 'dense' stitch function. If some keys are missing it will stitch things as close as is possible (it stitches based on key order rather than the value/true extent of keys). It assumes that every possible key in some region will exist in the iterator being stitched into a single tile.

We need to add a stitch function that will work correctly even with missing / skipped keys - a 'sparse' stitch

[pomadchin]

The implementation is here, it would be good to come up with the idea how to move it into GT core.