gpu: make Instance.requestAdapter friendlier

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

gpu/src/instance.zig

@@ -15,8 +15,34 @@ pub const Instance = opaque {
         return Impl.instanceCreateSurface(instance, descriptor);
     }
 
-    pub inline fn requestAdapter(instance: *Instance, options: *const RequestAdapterOptions, callback: RequestAdapterCallback, userdata: ?*anyopaque) void {
+    pub inline fn requestAdapter(
-        Impl.instanceRequestAdapter(instance, options, callback, userdata);
+        instance: *Instance,
+        options: *const RequestAdapterOptions,
+        comptime Context: type,
+        comptime callback: fn (
+            status: RequestAdapterStatus,
+            adapter: *Adapter,
+            message: ?[*:0]const u8,
+            ctx: Context,
+        ) callconv(.Inline) void,
+        context: Context,
+    ) void {
+        const Helper = struct {
+            pub fn callback(
+                status: RequestAdapterStatus,
+                adapter: *Adapter,
+                message: ?[*:0]const u8,
+                userdata: ?*anyopaque,
+            ) callconv(.C) void {
+                callback(
+                    status,
+                    adapter,
+                    message,
+                    if (Context == void) {} orelse @ptrCast(Context, userdata),
+                );
+            }
+        };
+        Impl.instanceRequestAdapter(instance, options, Helper.callback, if (Context == void) null orelse context);
     }
 
     pub inline fn reference(instance: *Instance) void {