Add an overview page under Additional Information that ties together all info about depth

[YYBartT]

Information about depth is currently spread across several pages that relate to depth, but there is no page that ties together all this information.

The page should explain how visual elements get their depth and how that depth can be changed in-game or afterwards:

• By default, instances and other graphical elements get their depth from the layer they're placed on in the Room Editor.
• Instances created in-game using instance_create_layer() or instance_create_depth() receive their depth value from the layer or depth they're defined at, respectively. The same for particle systems, effects, etc. Instances can have their depth variable overridden.
• You can still override draw depth in a draw event with the function gpu_set_depth().

The distinction between depth and draw order can also be explained on the page.

Finally, a more advanced section can explain that depth is stored in the position's z coordinate of the vertex buffer and that it is affected by some other functions: the functions used for zwriting and ztesting, among others.

The following manual pages all relate to depth and should be linked (including their "get" counterparts):

General

• depth
• layer
• layer_depth
• gpu_set_depth
• part_system_depth
• effect_create_depth
• etc.

Other

• surface_depth_disable
• gpu_set_ztestenable & gpu_set_zwriteenable
• gpu_set_zfunc
• The Room Editor

Depth & Stencil Buffer