gpu: add CommandEncoder.Descriptor

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

gpu/src/command_encoder.zig

@@ -1,6 +1,13 @@
+const ChainedStruct = @import("types.zig").ChainedStruct;
+
 pub const CommandEncoder = enum(usize) {
     _,
 
     // TODO: verify there is a use case for nullable value of this type.
     pub const none: CommandEncoder = @intToEnum(CommandEncoder, 0);
+
+    pub const Descriptor = struct {
+        next_in_chain: *const ChainedStruct,
+        label: ?[*:0]const u8 = null,
+    };
 };