Account for padding of structure fields in MSL backend

[zesterer]

I updated Naga to the latest commit 2 days ago. My WGSL shader parses and passes validation, but produces the following error on Mac OS (i.e: when Naga attempts to general MSL):

[2021-07-06T17:59:47Z ERROR wgpu::backend::direct] wgpu error: Validation Error
    
    Caused by:
        In Device::create_render_pipeline
          note: label = `Render pipeline`
        Internal error in VERTEX shader: Error compiling the shader "
"Compilation failed:
program_source:151:66: error: cannot initialize an array element of type 'char' with an lvalue of type 'float3' (vector of 3 'float' values)
    VertexOut _e57 = VertexOut {_e52 * _e50, _e45.norm, _e45.uv, _e50.xyz};
                                                                 ^~~~~~~~
program_source:151:66: warning: suggest braces around initialization of subobject
    VertexOut _e57 = VertexOut {_e52 * _e50, _e45.norm, _e45.uv, _e50.xyz};
                                                                 ^~~~~~~~
                                                                 {       }
program_source:176:51: error: cannot initialize an array element of type 'char' with an lvalue of type 'metal::float3' (aka 'float3')
    VertexOut _e46 = VertexOut {_e42, _e43, _e44, _e45};
                                                  ^~~~
program_source:176:51: warning: suggest braces around initialization of subobject
    VertexOut _e46 = VertexOut {_e42, _e43, _e44, _e45};
                                                  ^~~~
                                                  {   }
"

The code I'm using is:

#include shaders.util.globals
#include shaders.util.light

[[block]]
struct Locals {
    m: mat4x4<f32>;
};
[[group(1), binding(0)]] var<uniform> locals: Locals;

struct VertexIn {
	[[location(0)]] pos: vec3<f32>;
	[[location(1)]] norm: vec3<f32>;
	[[location(2)]] uv: vec2<f32>;
};

struct VertexOut {
	[[builtin(position)]] pos: vec4<f32>;
	[[location(0)]] norm: vec3<f32>;
	[[location(1)]] uv: vec2<f32>;
	[[location(2)]] wpos: vec3<f32>;
};

[[stage(vertex)]]
fn vs_main(vert: VertexIn) -> VertexOut {
	let wpos = locals.m * vec4<f32>(vert.pos, 1.0);
	return VertexOut(
		globals.vp * wpos,
		vert.norm,
		vert.uv,
		wpos.xyz,
	);
}

[[group(1), binding(1)]] var color_texture: texture_2d<f32>;
[[group(1), binding(2)]] var color_sampler: sampler;

[[stage(fragment)]]
fn fs_main(frag: VertexOut) -> [[location(0)]] vec4<f32> {
	let albedo = textureSample(color_texture, color_sampler, frag.uv).rgb;
	return vec4<f32>(illuminate(frag.wpos, normalize(frag.norm), albedo, 0.5, 1.0), 1.0);
}

(Ignore the #include pragmas, they're gone by the time the code reaches Naga)

This issue can be 'fixed' by making all but the last field of VertexOut a similar size, like so:

struct VertexOut {
	[[builtin(position)]] pos: vec4<f32>;
	[[location(0)]] norm: vec4<f32>;
	[[location(1)]] uv: vec4<f32>;
	[[location(2)]] wpos: vec3<f32>;
};

[teoxoy]

This has been fixed by #1215