app,app/internal/wm: implement ViewEvent for macOS

Move the deprecated setWantsBestResolutionOpenGLSurface to GL-specific
code while we're here.

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

app/app.go

@@ -9,6 +9,12 @@ import (
 	"gioui.org/app/internal/wm"
 )
 
+// ViewEvent carries the platform specific window handles for
+// a Window.
+//
+// ViewEvent is implemented on Android and macOS.
+type ViewEvent = wm.ViewEvent
+
 // extraArgs contains extra arguments to append to
 // os.Args. The arguments are separated with |.
 // Useful for running programs on mobiles where the