Add WgpuConfiguration::desired_maximum_frame_latency (#3874)

Setting `desired_maximum_frame_latency` to a low value should
theoretically lead to lower latency in winit apps using `egui-wgpu`
(e.g. in `eframe` with `wgpu` backend).

* Replaces https://github.com/emilk/egui/pull/3714
* See also https://github.com/gfx-rs/wgpu/pull/4899

----

It seems like `desired_maximum_frame_latency` has no effect on my Mac. I
lowered my monitor refresh-rate to 30Hz to test, and can see no
difference between `desired_maximum_frame_latency` of `0` or `3`.

Before when experimenting with changing the global `DESIRED_NUM_FRAMES`
in `wgpu` I saw a huge difference, so I wonder what has changed.

I verified that `set_maximum_drawable_count` is being called with either
`1` or `2`, but I perceive no difference between the two.

Cargo.lock

@@ -4219,9 +4219,9 @@ checksum = "14247bb57be4f377dfb94c72830b8ce8fc6beac03cf4bf7b9732eadd414123fc"
 
 [[package]]
 name = "wgpu"
-version = "0.19.0"
+version = "0.19.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d0b71d2ded29e2161db50ab731d6cb42c037bd7ab94864a98fa66ff36b4721a8"
+checksum = "0bfe9a310dcf2e6b85f00c46059aaeaf4184caa8e29a1ecd4b7a704c3482332d"
 dependencies = [
  "arrayvec",
  "cfg-if",