Add missing implementation of Renderer::GetViewportSize().
Add missing implementation of Renderer::GetViewportSize().

This was causing Zooshi's Android build to fail.

Tested: Zooshi for Android builds now on Linux.
Change-Id: I21739503e5d924e6e8b96dcac2c3d6334b922e18

google/fplbase,google/fplbase,google/fplbase,google/fplbase