gpu: add ComputePipeline.Descriptor

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

gpu/src/compute_pipeline.zig

@@ -1,6 +1,17 @@
+const ChainedStruct = @import("types.zig").ChainedStruct;
+const ProgrammableStageDescriptor = @import("types.zig").ProgrammableStageDescriptor;
+const PipelineLayout = @import("pipeline_layout.zig").PipelineLayout;
+
 pub const ComputePipeline = enum(usize) {
     _,
 
     // TODO: verify there is a use case for nullable value of this type.
     pub const none: ComputePipeline = @intToEnum(ComputePipeline, 0);
+
+    pub const Descriptor = extern struct {
+        next_in_chain: *const ChainedStruct,
+        label: ?[*:0]const u8 = null,
+        layout: PipelineLayout = PipelineLayout.none, // nullable
+        compute: ProgrammableStageDescriptor,
+    };
 };