Viewer: keyboard shortcut, write output for GLSL Vulkan Shader source

[juandejoya]

Addressing issue #2380 with the following implementations:

• Added a Vulkan shader generator context that follows the pattern of the GLSL shader generator context. This is used to write write out the Vulkan shader source to disk in SPIR-V.
• The Viewer uses a shader generator's unique target identifier to determine how it should write out a shader source to disk. We update VkShaderGenerator to have a unique vulkan target and added a vulkan.mtlx target definition.
• Added a keyboard shortcut (V) to write out the GLSL Vulkan shader source and updated the help UI.

The following image shows the behavior exhibited in this PR:

[juandejoya]

Looking at Matrix: build failures, they seem to be related copying/moving being prohibited for the GenContext constructor:

GenContext() = delete;

Not sure if this is already expected behavior from the code (that may need to be fixed?) since the the GenContext consturctor similarly across the different shader generators. I imagine I should be using references or pointers to address the solution, but I may be missing something.

[kwokcb]

Looking at Matrix: build failures, they seem to be related copying/moving being prohibited for the GenContext constructor:

GenContext() = delete;

Not sure if this is already expected behavior from the code (that may need to be fixed?) since the the GenContext consturctor similarly across the different shader generators. I imagine I should be using references or pointers to address the solution, but I may be missing something.

The context is meant hold / reference per target (generator) specific data so per target you can reuse but not between different targets. GenOptions are the reusable part.

[kwokcb]

I will suggest any renaming of the target for Vulkan be put into a separate issue and change where we can perform suitable testing.

[jstone-lucasfilm]

@juandejoya Just following up on this PR, to see if you might have the bandwidth to address the build issues!

[juandejoya]

@jstone-lucasfilm thanks for poking! I was going to reach out myself for how to proceed since it does require setting the target definition. I'll look into the build issues this week.