all: [API] change clip.RRect and UniformRRect to take integer coordinates

Like the change to op.Offset before this, clip.RRect and UniformRRect
is usually used with integer coordinates. Change to integer coordinates
to eliminate many useless conversions to float32.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/material/checkable.go

@@ -6,7 +6,6 @@ import (
 	"image"
 	"image/color"
 
-	"gioui.org/f32"
 	"gioui.org/internal/f32color"
 	"gioui.org/layout"
 	"gioui.org/op/clip"
@@ -50,7 +49,7 @@ func (c *checkable) layout(gtx layout.Context, checked, hovered bool) layout.Dim
 
 					background := f32color.MulAlpha(c.IconColor, 70)
 
-					b := f32.Rectangle{Max: f32.Pt(float32(size), float32(size))}
+					b := image.Rectangle{Max: image.Pt(size, size)}
 					paint.FillShape(gtx.Ops, background, clip.Ellipse(b).Op(gtx.Ops))
 
 					return dims