DispatchLoader: Allow passing explicit vkGetDeviceProcAddr to dynamic dispatch loader

[M2-TE]

This will resolve #2578.

This function signature overload for DispatchLoaderDynamic::init previously did not do anything with the vkGetDeviceProcAddr param:

// This interface does not require a linked vulkan library.
void init( VkInstance                instance,
           PFN_vkGetInstanceProcAddr getInstanceProcAddr,
           VkDevice                  device              = {},
           PFN_vkGetDeviceProcAddr /*getDeviceProcAddr*/ = nullptr ) VULKAN_HPP_NOEXCEPT
{
  VULKAN_HPP_ASSERT( instance && getInstanceProcAddr );
  vkGetInstanceProcAddr = getInstanceProcAddr;
  init( Instance( instance ) );
  if ( device )
  {
    init( Device( device ) );
  }
}

This PR does 4 things:

1. Changed aforementioned function to now use the vkGetDeviceProcAddr parameter
2. Removed default parameter assignments from aforementioned function
3. Added function overload that only takes instance and vkInstanceProcAddr
4. Removed surplus load of vkGetDeviceProcAddr in void init( Device deviceCpp )

I'm marking this as draft for now, as CI does not run these tests and thus, this change could be dangerous without proper testing. Please give it a try and see if it works for you @PancakeTAS