Different anchors for Stamps #3250

Multi-tile Stamps are currently always placed centered on the cursor. This can be a problem when trying to place stamps with variants of differing sizes precisely. For example, if I want to draw a line of trees, the shorter trees will be placed higher than the taller trees.

For stamps like this, it would be very helpful to be able to set an anchor point, so that I could choose e.g. bottom center for trees.

Since this is only relevant when using saved stamps with variants, perhaps anchors could be saved per stamp/variant, using a dropdown like Tilesets' Object Alignment, or a graphical selector: At a minimum, I think the options should be Center, since that's the usual behaviour, and Bottom, as that's very common in both sidescrollers and top-down games, useful for any stamps that are attached to the ground. Other anchors are useful less often, but should still be offered if it's not too difficult. Saving this value on a per-stamp basis means that the user won't have to change any tool properties when they switch swamps, and won't have to think about alignment at all when it doesn't matter.

The default value for stamps should be Center to match with the current behaviour, and newly added variants should default to the value used to the previous stamp/variant within that stamp.