gpu: add BindGroupLayout.Descriptor

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu/src/bind_group_layout.zig

@@ -20,4 +20,11 @@ pub const BindGroupLayout = enum(usize) {
         texture: Texture.BindingLayout,
         storage_texture: StorageTextureBindingLayout,
     };
+
+    pub const Descriptor = extern struct {
+        next_in_chain: *const ChainedStruct,
+        label: ?[*:0]const u8 = null,
+        entry_count: u32,
+        entries: [*]const Entry,
+    };
 };