Emit layout blocks for stage interfaces in Vulkan

[ashwinbhat]

The VkShaderGenerator was earlier flattening the VertexData into separate layout entries. While this is valid syntax, it is preferred to use the same struct as glsl to be consistent.

current emitted code:

layout (location = 0) out vec3 normalWorld;
layout (location = 1) out vec3 tangentWorld;
layout (location = 2) out vec3 positionWorld;

new proposed output:

layout (location = 0) out VertexData
{
    vec3 normalWorld;
    vec3 tangentWorld;
    vec3 positionWorld;
} vd;

Also fixes #2271

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: jstone-lucasfilm / name: Jonathan Stone (ef078d0, 230e703, 699c883, 41b3361)
• ✅ login: ashwinbhat / name: Ashwin Bhat (99b0811, 7a71ea8, a73332e, 0a778bf, 8241961)

[jstone-lucasfilm]

This change looks good to me, @ashwinbhat, and I'm CC'ing @kwokcb for his thoughts.

[ashwinbhat]

@jstone-lucasfilm @bernardkwok
I've updated the yml to reenable vulkan validation and generation.