gesture,widget,f32: [API] use integer coordinates for gesture coordinates

Most widget code operate in integer coordinates. This change makes
gesture pointer coordinates integer, to lessen the number of float32
to int conversions.

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

widget/button.go

@@ -6,7 +6,6 @@ import (
 	"image"
 	"time"
 
-	"gioui.org/f32"
 	"gioui.org/gesture"
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
@@ -39,7 +38,7 @@ type Click struct {
 // Press represents a past pointer press.
 type Press struct {
 	// Position of the press.
-	Position f32.Point
+	Position image.Point
 	// Start is when the press began.
 	Start time.Time
 	// End is when the press was ended by a release or cancel.

widget/material/button.go

@@ -297,7 +297,7 @@ func drawInk(gtx layout.Context, c widget.Press) {
 	ink := paint.ColorOp{Color: rgba}
 	ink.Add(gtx.Ops)
 	rr := size * .5
-	defer op.Offset(c.Position.Add(f32.Point{
+	defer op.Offset(layout.FPt(c.Position).Add(f32.Point{
 		X: -rr,
 		Y: -rr,
 	})).Push(gtx.Ops).Pop()