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,
 };

gpu/src/pipeline_layout.zig

@@ -1,16 +1,11 @@
 const ChainedStruct = @import("types.zig").ChainedStruct;
 const BindGroupLayout = @import("bind_group_layout.zig").BindGroupLayout;
 
-pub const PipelineLayout = enum(usize) {
+pub const PipelineLayout = *opaque {};
-    _,
 
-    // TODO: verify there is a use case for nullable value of this type.
+pub const PipelineLayoutDescriptor = extern struct {
-    pub const none: PipelineLayout = @intToEnum(PipelineLayout, 0);
+    next_in_chain: *const ChainedStruct,
-
+    label: ?[*:0]const u8 = null,
-    pub const Descriptor = extern struct {
+    bind_group_layout_count: u32,
-        next_in_chain: *const ChainedStruct,
+    bind_group_layouts: [*]const BindGroupLayout,
-        label: ?[*:0]const u8 = null,
-        bind_group_layout_count: u32,
-        bind_group_layouts: [*]const BindGroupLayout,
-    };
 };

gpu/src/render_pipeline.zig

@@ -15,7 +15,7 @@ pub const RenderPipeline = enum(usize) {
     pub const Descriptor = extern struct {
         next_in_chain: *const ChainedStruct,
         label: ?[*:0]const u8 = null,
-        layout: PipelineLayout = PipelineLayout.none, // nullable
+        layout: ?PipelineLayout,
         vertex: VertexState,
         primitive: PrimitiveState,
         depth_stencil: ?*const DepthStencilState = null, // nullable