GPU : Document the NDC

[myriadbytes]

Hi !
I can't find anything on the wiki about what NDC coordinate system the SDL3 GPU API uses.
At first I was wondering if SDL3 GPU even abstracted this, or if the user needed to modify their shaders depending on the backend used.

Looking at the discord, it seems like the API does have its own NDC, but the only info I have found is that +Y is definitely "up" .
There were also some messages about documenting this, but it doesn't seem to have led to anything.

It would be nice to clear this up somewhere in the wiki.

[lmurray]

From what I've been able to determine the following is what the GPU API uses:

The Normalized Device Coordinates (NDC) is a 2x2x1 box with its center at (0, 0, 0.5), spanning from (-1, -1, 0) to (1, 1, 1). Positive X is to the right, positive Y is upwards, and positive Z is away from the viewer. This is a left-handed coordinate system.

I am uncertain about the Z axis though so someone will need to verify that before it can be added to the wiki.

[lmurray]

Fixed by 8e0b39f, although it's slightly mangled right now.

[slouken]

Unmangled, thanks!