wgpu: Initialize InstanceDescriptor from env. (#774)

This makes the instance descriptor fully configurable from the
environment using the underlying wgpu mechanisms.

vello/src/util.rs

@@ -30,10 +30,16 @@ impl RenderContext {
         reason = "Creating a wgpu Instance is something which should only be done rarely"
     )]
     pub fn new() -> Self {
+        let backends = wgpu::util::backend_bits_from_env().unwrap_or_default();
+        let flags = wgpu::InstanceFlags::from_build_config().with_env();
+        let dx12_shader_compiler = wgpu::util::dx12_shader_compiler_from_env().unwrap_or_default();
+        let gles_minor_version = wgpu::util::gles_minor_version_from_env().unwrap_or_default();
+
         let instance = Instance::new(wgpu::InstanceDescriptor {
-            backends: wgpu::util::backend_bits_from_env().unwrap_or(wgpu::Backends::PRIMARY),
-            dx12_shader_compiler: wgpu::Dx12Compiler::Fxc,
-            ..Default::default()
+            backends,
+            flags,
+            dx12_shader_compiler,
+            gles_minor_version,
         });
         Self {
             instance,