Backends: vulkan: add possibility for custom frag and vertex shaders

[johan0A]

This is my fix I use for: #8271

[ocornut]

I am tempted to accept it (maybe with VkShaderModule inputs instead? so if vkCreateShaderModule() ever adds flags that'd be in control of the user), but it seems like it would be a better general-purpose feature to make it easier to change shaders in the render loop.

[johan0A]

Jus to clarify I dont think this really solves #8271 but it does offer a good workaround for it.

(maybe with VkShaderModule inputs instead? so if vkCreateShaderModule() ever adds flags that'd be in control of the user)

Or with a VkShaderModuleCreateInfo?

[olivier-gerard]

Just adding my 2 cents : having custom VkShaderModuleCreateInfo fields look nice has it hosts user flags.
Sharing VkShaderModule objects with main app is more complex as they shall be destroyed and responsibility of this may be unclear.

[ocornut]

I have reworked this to use VkShaderModuleCreateInfo and merged as 01686c6.
Thanks both for your input!