Bottom center and other alignments for Tile Layers

[eishiya]

Tiles in Tile Layers are drawn with bottom left alignment in their cells. For isometric and hex maps, it's often more intuitive to draw them with bottom center alignment instead. In cases where the tiles vary in size, this alignment would allow one to reduce the amount of padding required around the tile, and possibly even remove the need to set drawing offsets on the tiles or tileset. This could benefit some orthographic tilesets as well, particularly 3/4 view prop tilesets that often have the prop centered horizontally in the tile.

Perhaps this could be a Tileset setting alongside Object Alignment, called "Tile Alignment", with all the same options, and "Unspecified" being the same as Bottom Left?

#871 could accomplish the same thing as this, but with more work for the user - they would have to manually set the paddings for each tile to center the tiles, whereas offering different alignments would be just one tileset-wide setting. Per-tile alignments could still be useful to reduce the required padding on extremely lop-sided tiles though, so these two feature requests are not mutually exclusive.

[VKM34]

My personnal use case of centered alignement is for asymetrical tile, so if centered can be used, there is no need of offsetting the tile because it would place it correctly directly.