`multi_draw_indirect` dispatches one draw call even when `count` is set to 0

[davidhi7]

Description
When calling RenderPass::multi_draw_indirect (or the indexed equivalent) with the count parameter set to 0, wgpu internally changes the draw count to 1.

Repro steps
See this sample gist: https://gist.github.com/davidhi7/47b35e393d83c0062f280737f765befb
This is effectively just the "hello-triangle" example changed to use indirect draw calls.

Expected vs observed behavior
If the count parameter is set to 0, then zero, and not one, draw calls should be dispatched.

Extra materials
I think the bug emerges here:

wgpu/wgpu-core/src/command/render.rs

Lines 3151 to 3156 in 959c2db

	base.commands.push(ArcRenderCommand::MultiDrawIndirect {
	buffer: self.resolve_render_pass_buffer_id(scope, buffer_id)?,
	offset,
	count: NonZeroU32::new(count),
	indexed: false,
	});

If count is 0, the command count field is set to None. Later, count: None is interpreted as a single indirect draw call:

wgpu/wgpu-core/src/command/render_command.rs

Lines 85 to 91 in 959c2db

	MultiDrawIndirect {
	buffer_id: id::BufferId,
	offset: BufferAddress,
	/// Count of `None` represents a non-multi call.
	count: Option<NonZeroU32>,
	indexed: bool,
	},

Platform
wgpu v23.0.1
Intel UHD Graphics 620
On Arch Linux running Vulkan

[cwfitzgerald]

Whoops was too fancy

[davidhi7]

Thanks for the quick fix, I didn't get around to test it until recently and unfortunately it seems this bug got reintroduced in some later version. I did a quick bisect and this commit appears to reintroduce the bug: ef622c1. Here is a minimal reproducible example: https://gist.github.com/davidhi7/8b18843267fb1931104660fe287843fd.