gpu: implement ExternalTexture

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

gpu/src/ExternalTexture.zig

@@ -0,0 +1,28 @@
+const ExternalTexture = @This();
+
+/// The type erased pointer to the ExternalTexture implementation
+/// Equal to c.WGPUExternalTexture for NativeInstance.
+ptr: *anyopaque,
+vtable: *const VTable,
+
+pub const VTable = struct {
+    reference: fn (ptr: *anyopaque) void,
+    release: fn (ptr: *anyopaque) void,
+    // TODO:
+    // WGPU_EXPORT void wgpuExternalTextureDestroy(WGPUExternalTexture externalTexture);
+    // WGPU_EXPORT void wgpuExternalTextureSetLabel(WGPUExternalTexture externalTexture, char const * label);
+};
+
+pub inline fn reference(texture: ExternalTexture) void {
+    texture.vtable.reference(texture.ptr);
+}
+
+pub inline fn release(texture: ExternalTexture) void {
+    texture.vtable.release(texture.ptr);
+}
+
+test "syntax" {
+    _ = VTable;
+    _ = reference;
+    _ = release;
+}