Rework

[adrien-ben]

Reworking the api. The RendererVkContext is pretty hard to work with especially when trying to integrate gpu-allocator.

• [x] Bump dependencies
• [x] ~Remove vkmem~
• [x] Remove the trait RendererVkContext
• [x] Integrate gpu-allocator
• [x] Update & cleanup docs
• [x] Add multiple fonts support
• [x] Update example (copied from https://github.com/imgui-rs/imgui-rs/tree/v0.8.2-patch/imgui-examples)

[adrien-ben]

I think i'm done with this but i'm still unsure about 2 things:

• when enabling gpu-allocator feature you need to create a Renderer using Renderer::with_gpu_allocator which takes a Arc<Mutex<gpu_allocator::vulkan::Allocator>>. It works for my usage of the renderer and you don't need to pass a reference to each function call. But I'm not sure it would feat other's usage of the crate or if it is good practice. Any suggestion is welcome.

• Should the gpu-allocator feature disable the default allocator ? It would allow me to get rid of the internal enum allowing multiple allocator implementation at the same time. It does not affect the api but still...

[adrien-ben]

I added vk-mem back but using a forked version for ash compatibility since original repo is not maintained anymore