min buffer sizes

[Greggman]

bring up minimum buffer sizes

Example

 struct Uniforms {
    lightDirection: vec3<f32>,
  };

  @group(0) @binding(0) var<uniform> uniforms1: Uniforms;
  @group(0) @binding(1) var<uinform> uniforms2: vec3<f32>;

WebGPU requires SizeOf(thing). According to the spec, SizeOf(vec3<f32>) is 12 where as SizeOf(Uniforms) is 16