feat: add renderer backend config option

[romanstingler]

Adds support for selecting Vulkan/OpenGL/Auto renderer backend via config.toml. Uses wgpu environment variables set before any initialization.

Automatic behavior:

• Auto mode (default): automatically uses OpenGL when proprietary NVIDIA driver is detected, Vulkan for all other hardware (AMD/Intel/nouveau)
• User can always explicitly select vulkan or opengl to override

Valid options:

• auto (default)
• vulkan - force Vulkan backend
• opengl - force OpenGL backend

related to discussion #450 (comment)

Testing:

add this to your config's top level
renderer_backend = "auto"

run

RUST_LOG=wgpu=debug cargo run -- -c config.toml > /tmp/log (or whatever config you use)

Vulkan

INFO [wgpu_core::instance] Adapter Vulkan AdapterInfo { name: "AMD Radeon 890M Graphics (RADV STRIX1)", vendor: 4098, device: 5390, device_type: IntegratedGpu, driver: "radv", driver_info: "Mesa 26.1.0-devel (git-d6958a5e43)", backend: Vulkan }

...
DEBUG [wgpu_core::present] Removing swapchain texture Id(6,37,vk) from the device tracker

and in OpenGL you will see

DEBUG [wgpu_core::present] Removing swapchain texture Id(6,30,gl) from the device tracker

@clotodex @MalpenZibo
It is the simplest I could come up with.