gpu: convert PipelineLayout from enum(usize) to *opaque

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

gpu/src/compute_pipeline.zig

@@ -7,6 +7,6 @@ pub const ComputePipeline = *opaque {};
 pub const ComputePipelineDescriptor = extern struct {
     next_in_chain: *const ChainedStruct,
     label: ?[*:0]const u8 = null,
-    layout: PipelineLayout = PipelineLayout.none, // nullable
+    layout: ?PipelineLayout,
     compute: ProgrammableStageDescriptor,
 };