gpu: add BindGroupLayout.Entry

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

gpu/src/bind_group_layout.zig

@@ -1,6 +1,23 @@
+const ChainedStruct = @import("types.zig").ChainedStruct;
+const ShaderStageFlags = @import("types.zig").ShaderStageFlags;
+const Buffer = @import("buffer.zig").Buffer;
+const Sampler = @import("sampler.zig").Sampler;
+const Texture = @import("texture.zig").Texture;
+const StorageTextureBindingLayout = @import("types.zig").StorageTextureBindingLayout;
+
 pub const BindGroupLayout = enum(usize) {
     _,
 
     // TODO: verify there is a use case for nullable value of this type.
     pub const none: BindGroupLayout = @intToEnum(BindGroupLayout, 0);
+
+    pub const Entry = extern struct {
+        next_in_chain: *const ChainedStruct,
+        binding: u32,
+        visibility: ShaderStageFlags,
+        buffer: Buffer.BindingLayout,
+        sampler: Sampler.BindingLayout,
+        texture: Texture.BindingLayout,
+        storage_texture: StorageTextureBindingLayout,
+    };
 };