Fix viewport scaling bug on android after context rebind (#2703)
On android after rebinding context (which happens when you minimize and
navigate back to an app, or when you turn a screen off and back on)
there's a bug that viewport has a wrong scale and part of it is off
screen.
The change fixes it on devices I tried, but the solution feels hacky to
me. However when I attempted instead to call SetupViewport() again which
feels like a more proper solution, it didn't fix it.