Postpone call to get_current_texture (#3914)

This should help slightly with CPU/GPU parallelism when vsync is on.

I also return the time spent on vsync, which can help users figure out
how much CPU wall-time was used on non-vsync stuff

crates/egui-wgpu/Cargo.toml

@@ -45,6 +45,7 @@ bytemuck = "1.7"
 log = { version = "0.4", features = ["std"] }
 thiserror.workspace = true
 type-map = "0.5.0"
+web-time.workspace = true
 wgpu = { workspace = true, features = ["wgsl"] }
 
 #! ### Optional dependencies