Save Tile Stack

[DanielJDufour]

If we save the reproject/resampled tile data (before band expression is applied), then a user can quickly apply new band expressions without needing to refetch/recreate the tile data.

[DanielJDufour]

This would involve refactoring updateColors, so that we don't have to redraw the tiles from scratch, only reapplying band arithmetic on the stack.