PaintCallbackInfo::viewport_in_pixels now guarantees to stay in bounds (#3604)

* Fixes https://github.com/rerun-io/rerun/issues/4297
* tested against a very hasty and incomplete port of egui/trunk, found
[here](https://github.com/rerun-io/rerun/tree/andreas/experimental-egui-trunk)


In rare cases in can happen that the viewport returned by
`PaintCallbackInfo` is outside the bounds of the screen.
for at least [wgpu/webgpu in
particular](https://www.w3.org/TR/webgpu/#dom-gpurenderpassencoder-setviewport)
this is invalid usage, other backends might be affected as well.
Since this happened due to a float rounding error (in one repro case I
had I got (width==1126.5625) + (offset=715.4376) = 1842.0001 for a
resolution of 1842) I decided to do away with fractional values on the
viewport alltogether. They _technically_ make sense since a viewport is
only specifying the NDC to pixel coordinate conversion, but practically
this may lead to surprising sub-sampling issues.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_glow/src/painter.rs

@@ -404,10 +404,10 @@ impl Painter {
                         let viewport_px = info.viewport_in_pixels();
                         unsafe {
                             self.gl.viewport(
-                                viewport_px.left_px.round() as _,
-                                viewport_px.from_bottom_px.round() as _,
-                                viewport_px.width_px.round() as _,
-                                viewport_px.height_px.round() as _,
+                                viewport_px.left_px,
+                                viewport_px.from_bottom_px,
+                                viewport_px.width_px,
+                                viewport_px.height_px,
                             );
                         }