eframe: window transparency doesn't work with wgpu

[emilk]

On my Mac, the eframe::NativeOptions::transparent option doesn't do anything.

Repro:

--- a/examples/custom_window_frame/Cargo.toml
+++ b/examples/custom_window_frame/Cargo.toml
@@ -9,6 +9,8 @@ publish = false
 
 
 [dependencies]
-eframe = { path = "../../crates/eframe", features = [
-    "__screenshot", # __screenshot is so we can dump a ascreenshot using EFRAME_SCREENSHOT_TO
+eframe = { path = "../../crates/eframe", default-features = false, features = [
+    "default_fonts",
+    "wgpu",
 ] }