Metal: Bind index buffer with offset

[pkdawson]

Fixes #96348

Referring to the Metal docs, indexBufferOffset is a byte offset, so p_first_index has to be converted.

[akien-mga]

CC @stuartcarnie

[stuartcarnie]

Ahh great find – thank you!

[stuartcarnie]

I tested and confirmed it works. Thanks @pkdawson!

[stuartcarnie]

I did notice a separate issue that the frame buffer and attachment formats do not match, which crash when enabling Metal validation. The fix was to update your script, when creating the render pipeline:

		RD.framebuffer_get_format(framebuffer),

vs

		RD.screen_get_framebuffer_format(),

[pkdawson]

Oh yeah, that's actually an issue I fixed pretty recently in my real code (Dear ImGui backend). I'm creating a framebuffer that uses RGBA, but Godot's screen format is ABGR (I think), so there's a validation error.

[akien-mga]

Thanks!