event/{mouse,touch}: work in float32 pixels, not geom.Pt.

Higher-level widget or animation libraries should probably work in
geom.Pt, but pixels instead of (1/72s of) inches seems a better fit for
lower-level event libraries. Needlessly converting from (float32) pixels
to (float32) points and back can be lossy and lead to off-by-one errors.

Change-Id: I68102e36f2574b07b44c6a1b7281f4f27f9174cf
Reviewed-on: https://go-review.googlesource.com/13002
Reviewed-by: David Crawshaw <crawshaw@golang.org>

example/basic/main.go

@@ -40,7 +40,6 @@ import (
 	"golang.org/x/mobile/exp/app/debug"
 	"golang.org/x/mobile/exp/f32"
 	"golang.org/x/mobile/exp/gl/glutil"
-	"golang.org/x/mobile/geom"
 	"golang.org/x/mobile/gl"
 )
 
@@ -51,8 +50,9 @@ var (
 	color    gl.Uniform
 	buf      gl.Buffer
 
-	green    float32
-	touchLoc geom.Point
+	green  float32
+	touchX float32
+	touchY float32
 )
 
 func main() {
@@ -69,12 +69,14 @@ func main() {
 				}
 			case config.Event:
 				c = e
-				touchLoc = geom.Point{c.WidthPt / 2, c.HeightPt / 2}
+				touchX = float32(c.WidthPx / 2)
+				touchY = float32(c.HeightPx / 2)
 			case paint.Event:
 				onPaint(c)
 				a.EndPaint(e)
 			case touch.Event:
-				touchLoc = e.Loc
+				touchX = e.X
+				touchY = e.Y
 			}
 		}
 	})
@@ -117,7 +119,7 @@ func onPaint(c config.Event) {
 	}
 	gl.Uniform4f(color, 0, green, 0, 1)
 
-	gl.Uniform2f(offset, float32(touchLoc.X/c.WidthPt), float32(touchLoc.Y/c.HeightPt))
+	gl.Uniform2f(offset, touchX/float32(c.WidthPx), touchY/float32(c.HeightPx))
 
 	gl.BindBuffer(gl.ARRAY_BUFFER, buf)
 	gl.EnableVertexAttribArray(position)