gpu: add ExternalTexture.BindingEntry

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

gpu/src/external_texture.zig

@@ -1,6 +1,13 @@
+pub const ChainedStruct = @import("types.zig").ChainedStruct;
+
 pub const ExternalTexture = enum(usize) {
     _,
 
     // TODO: verify there is a use case for nullable value of this type.
     pub const none: ExternalTexture = @intToEnum(ExternalTexture, 0);
+
+    pub const BindingEntry = extern struct {
+        chain: ChainedStruct,
+        external_texture: ExternalTexture,
+    };
 };