gpu: add SwapChain.Descriptor

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

gpu/src/swap_chain.zig

@@ -1,6 +1,21 @@
+const ChainedStruct = @import("types.zig").ChainedStruct;
+const PresentMode = @import("types.zig").PresentMode;
+const Texture = @import("texture.zig").Texture;
+
 pub const SwapChain = enum(usize) {
     _,
 
     // TODO: verify there is a use case for nullable value of this type.
     pub const none: SwapChain = @intToEnum(SwapChain, 0);
+
+    pub const Descriptor = extern struct {
+        next_in_chain: *const ChainedStruct,
+        label: ?[*:0]const u8 = null,
+        usage: Texture.UsageFlags,
+        format: Texture.Format,
+        width: u32,
+        height: u32,
+        present_mode: PresentMode,
+        implementation: u64,
+    };
 };