Fix broken rendering of tilemaps with padding in atlas mode

[rparrett]

Fixes #352

var columns was mistakenly adding rather than subtracting one side of the spacing, potentially causing the wrong tiles to be displayed. This bug was not visible in the spacing example. However, this would have produced the wrong result when spacing was greater than or equal to tile size.

sprite_sheet_x and sprite_sheet_y were not accounting for the what bevy calls the "offset" -- the spacing in the left gutter of the texture.

Before:
cargo run --example spacing --features=atlas

After
cargo run --example spacing --features=atlas

Note: the spacing example is intentionally displaying an incorrectly configured tilemap in blue.

[bzm3r]

@rparrett thanks for your patience on this; I've asked StarArawn to review, but he is currently super busy, so it may take a while!

But this does look good to me!

[StarArawn]

@rparrett Can you add a comment? Once that's done I'll get this merged! :)