Tileset Editor: Allow the first tile in a tileset to not be an empty/null tile

[iampremo]

Requested in Ticket #205314

Feature Type: feat_change_ide Description: Pretty much since Game Maker first used tilesets, the very first tile has always been a null tile. This has been a complete pain as literally no tileset ever made (that I've found anyway) does that. I simply request that this be changed. There is no good reason that I can think of why it is the way it currently is. Benefit: This would allow for importing tilesets from literally anywhere to become the simple process it should have always been. Instead, right now I have to manually rearrange tilesets to fit the way Game Maker does it. Be that going into an image editor and moving things around or the easier but ugly method of just having the top most row of tiles be completely blank. This wastes time and space. Changing the null tile from being at the start to either at the end or simply not there at all simplifies things for everyone.

[colinwilcox]

As someone with a relatively large library of existing tilesets in a "normal" format this makes using GM pretty frustrating.