Additional suggestions for 0.3.0 improvements

[leprechaunt33]

• Make SwapchainInfo::from_openxr_swapchain a pub fn
  Rationale: it is useful sometimes to create your own custom renderer to mimic the actions taken in the default render context, or to work with it.

In my example code I used:

    let swapchain_info = SwapchainInfo::from_openxr_swapchain(&xr_context.swapchain, render_area

.extent).unwrap();

And then iterated the swapchain images to ensure I was working in accordance with the already existing aspects of the swapchain. This is invaluable for anyone writing their own custom renderer.

• Texture sampler woes

In vulkan_context.rs, the texture sampler is created with anisotropy off and lod settings which are useful for a narrow segment of working apps. It would be useful to be able to specify alternate parameters for the default texture sampler, including most pressingly the border color and address mode for the sampler, and optionally the LOD to avoid the quite noticeable moire patterns and other problems created by the default values used by the min and mag filters.

• Improved mipmap support

in Texture::empty, I found myself creating a Texture::empty_no_mip, so Texture::empty could have its default or specified mip levels, while another could be for unlit workflows without mipmapping. It would also be super helpful to be able to specify a name for the "empty" textures, for debugging purposes, rather than every single one of them being called "Empty Texture".

There are the beginnings of mipmap support in vulkan_context.rs, but it would be good to flesh these out with some kind of functionality to enable linear copy of an image into its mip levels to auto-generate mips, something like what is available in example 28 in Vulkanalia.

I also made changes to the size of the vertex and index buffers in rendering/resources.rs... you already indicated this in code as a TODO, but it would be nice to be able to configure a size for these.