Add ZIndex property to SurfaceAppearance

As a Roblox developer, it is currently too hard to allow in-depth customization of MeshParts. If this issue were addressed, it would allow us to have some in game content like the following example:

Car customization example:
Imagine a game with 100 different cars. We allow the player to customize their car to their liking - this includes whatever kind of spray paint job and image decorations you can imagine. Maybe you want flames or a checkered pattern, waves, sparkles, shine, rust. And maybe you’d like to add a combination of cool or funny decorations like emojis, numbers, words, skulls, lightning, dents, scratches, and tons of other sorts of emblems and pictures to your car. The fact that these images can overlap ontop of eachother, and that they will be able to apply to 100 different car types (MeshParts), means, we need Z Index. One may argue that we can just duplicate a dozen of the car MeshParts, make it invisible and offset it slightly to achieve a Z-Depth for every added decoration, but the reality is, resizing any abstract object means that the resized objects will cut each other off and not fit correctly within each other.

This kind of customization would apply much more often and be more beneficial for things like character customization. And of course there are plenty of other miscellaneous use cases, like guns/weapon customization, building/house customization, ingame content creation like ingame UGC, custom worlds/objects, etc, all applicable in a similar manner to how the described car customization works.

In summary, without stating our desired feature solution: We want to allow players to customize thousands of MeshParts, with a pool of thousands of images, any of which can be combined on the same part.

In addition, ultimately the same use cases that were stated for supporting Z Index for Decals and Textures, which has now been approved and implemented into the game engine, are the same use cases for supporting it within SurfaceAppearance.

Without this, much of our project is not able to be made yet, and over time I’m hoping to convince everyone of the benefit of this feature. Thanks for reading.