Make ViewportInPixels use integers, and clamp to bounds

[Wumpf]

• Fixes wgpu 0.18 warning spam when resizing window rerun-io/rerun#4297
• tested against a very hasty and incomplete port of egui/trunk, found here

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 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.

[emilk]

Keeping in bounds makes sense, and rounding to integers makes sense too.
I’m not sure it should be epaints job, but since this will likely be a footgun for many backend, maybe it should.

There is a small detail of how to round.
We want two adjacent Rect (sharing an edge) to have non-overlapping viewports with no gap or overdraw. For that I think we need to round the edges (right/bottom) rather than the sizes (width/height).

Also, if epaint is always rounding, then ViewportInPixels should use i32 instead of f32