fix: blit should only draw one triangle

[CurryPseudo]

Description
Original shader:

wgpu/wgpu/examples/mipmap/blit.wgsl

Lines 7 to 22 in 48f8811

	fn vs_main(@builtin(vertex_index) vertex_index: u32) -> VertexOutput {
	var out: VertexOutput;
	let x = i32(vertex_index) / 2;
	let y = i32(vertex_index) & 1;
	let tc = vec2<f32>(
	f32(x) * 2.0,
	f32(y) * 2.0
	);
	out.position = vec4<f32>(
	tc.x * 2.0 - 1.0,
	1.0 - tc.y * 2.0,
	0.0, 1.0
	);
	out.tex_coords = tc;
	return out;
	}

x in [0, 1]
y in [0, 1]
out.tex_coord in [(0, 0), (2, 2)]
out.position in [(-1, -3), (3, 1)]
Clearly it is wrong, but it actually covers the whole render area([(-1, -1), (1, 1)]), and tex_coord happened to be right, so it worked.

Testing
Run the mipmap example, result is the same.
master branch:

this branch:

[kvark]

This is actually by design. The shader is written as if one large triangle is drawn.
The confusing part is that the Rust side is written to spew 2 triangles in a strip. I think that part needs to be fixed to just use one triangle as triangle list instead.

[CurryPseudo]

That makes sense.
I changed to triangle list and use only 3 vertices, it works.

[kvark]

Thank you!