sixel: don’t crash when sixel image exceeds current sixel max height

sixel: don’t crash when sixel image exceeds current sixel max height When we try to resize a sixel past the current max height, we set col > image-width to signal this. This means ‘width’ could be smaller than ‘col’. When calculating how many sixels to emit in sixel_add_many(), we didnt’ account for this. The resulting value was -1, converted to ‘unsigned’. I.e. a very large value. This resulted in an assert triggering in sixel_add() in debug builds, and a crash in release builds.