ui,ui/app: convert Config to an interface

To keep the interface slim, remove the helper methods and shorten
the essential method, Pixels, to Px.

Add and use unexported Config implementation in the app package.

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

ui/app/os_windows.go

@@ -13,7 +13,6 @@ import (
 
 	syscall "golang.org/x/sys/windows"
 
-	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -220,8 +219,8 @@ func createNativeWindow(opts *WindowOptions) (*window, error) {
 	}
 	defer unregisterClass(cls, hInst)
 	wr := rect{
-		right:  int32(cfg.Pixels(opts.Width)),
-		bottom: int32(cfg.Pixels(opts.Height)),
+		right:  int32(cfg.Px(opts.Width)),
+		bottom: int32(cfg.Px(opts.Height)),
 	}
 	dwStyle := uint32(_WS_OVERLAPPEDWINDOW)
 	dwExStyle := uint32(_WS_EX_APPWINDOW | _WS_EX_WINDOWEDGE)
@@ -419,7 +418,7 @@ func (w *window) draw(sync bool) {
 	w.width = int(r.right - r.left)
 	w.height = int(r.bottom - r.top)
 	cfg := configForDC(w.hdc)
-	cfg.Now = time.Now()
+	cfg.now = time.Now()
 	w.w.event(DrawEvent{
 		Size: image.Point{
 			X: w.width,
@@ -487,16 +486,16 @@ func convertKeyCode(code uintptr) (rune, bool) {
 	return r, true
 }
 
-func configForDC(hdc syscall.Handle) ui.Config {
+func configForDC(hdc syscall.Handle) Config {
 	dpi := getDeviceCaps(hdc, _LOGPIXELSX)
 	ppdp := float32(dpi) * inchPrDp * monitorScale
 	// Force a minimum density to keep text legible and to handle bogus output geometry.
 	if ppdp < minDensity {
 		ppdp = minDensity
 	}
-	return ui.Config{
-		PxPerDp: ppdp,
-		PxPerSp: ppdp,
+	return Config{
+		pxPerDp: ppdp,
+		pxPerSp: ppdp,
 	}
 }