Adding android-like transitions

This pr adds two new shader transitions to the screenmanager, one where the old screen shrinks and becomes transparent to reveal the new one behind it, and another where the new screen grows from the screen centre to obscure the old one. These mimic the transitions that are quite popular in android apps.

The growth and opacity functions are guesses that look about right, I couldn't find any exact details of the shader used normally in android.

This could also be a garden package, but  I think it should go in kivy core because there's currently no transition that mimics the equivalent android ones.