Panic in web builds

[rparrett]

Problem

wasm.js:374 panicked at 'wgpu error: Validation Error

Caused by:
    In Device::create_render_pipeline
      note: label = `transparent_mesh2d_pipeline`
    In the provided shader, the type given for group 1 binding 0 has a size of 4. As the device does not support `DownlevelFlags::BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED`, the type must have a size that is a multiple of 16 bytes.

', /Users/me/.cargo/registry/src/github.com-1ecc6299db9ec823/wgpu-0.15.1/src/backend/direct.rs:3024:5

Reproduction

To reproduce, it may be simplest to use wasm-server-runner.

1. Follow the directions in the link above to install wasm-server-runner
2. cargo run --target wasm32-unknown-unknown --example tiling
3. Open your browser to the address given
4. Open javascript console to observe error

Additional Info

This is a minimal sort of fix that solves the issue. Obviously not ideal as BackgroundMaterial is pub.

For more background, here's a PR that fixed a similar issue in Bevy itself: bevyengine/bevy#6460

diff --git a/src/lib.rs b/src/lib.rs
index 996500e..8cdb948 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -149,6 +149,8 @@ pub trait ScrollingBackground {
 pub struct BackgroundMaterial {
     #[uniform(0)]
     pub movement_scale: f32,
+    #[uniform(0)]
+    pub _wasm_padding: Vec3,
     /// This image must have its [`SamplerDescriptor`] address_mode_* fields set to
     /// [`AddressMode::Repeat`].
     #[texture(1)]
@@ -302,6 +304,7 @@ impl BackgroundImageBundle {
             material: background_materials.add(BackgroundMaterial {
                 texture: image,
                 movement_scale: 1.0,
+                _wasm_padding: Vec3::ZERO,
             }),
             mesh: BG_MESH_HANDLE.typed().into(),
             transform: Default::default(),
diff --git a/src/shaders/background.wgsl b/src/shaders/background.wgsl
index 10942d5..bc0fff6 100644
--- a/src/shaders/background.wgsl
+++ b/src/shaders/background.wgsl
@@ -5,6 +5,7 @@
 #import braymatter::bglib
 struct Uniforms {
     scale: f32,
+    _wasm_padding: vec3<f32>
 };
 
 @group(1) @binding(0)