Write API for screen transitions

[bitbrain]

This feature allows transitions between 2 screens.

List of supported transitions

• FadeTransition fades the alpha value of the current screen to 0 and the alpha value of the new screen to 1 simultaneously.
• SwipeTransition moves the current screen to [top | left | right | bottom] and the next screen moves to the middle.
• ColorTransition fades the current screen to a color specified and fades from the color to the next screen.

Notes and considerations

• the buffer texture of the current screen can be obtained by the rendering pipeline
• the buffer texture of the next screen could be obtained by the rendering pipeline. However it may not be configured correctly. Additionally the pipeline may depend on assets which are not loaded. Make sure to support screen transitions for lazy-loading of assets later on!
• if a texture can not be obtained correctly, do not fade at all and show a warning in the log.

[bitbrain]

The existing API is sufficient for now.