Add default values for all parameters of the clearScreen function

[ImXico]

The clearScreen function inside core/Graphics.kt has a default value for the alpha parameter. It might be a good idea to also make the other three parameters optional. In other words, the red, green and blue components should also have default values.

The respective in-code documentation should be updated accordingly. Furthermore, the method should be marked with the @JvmOverloads annotation, to keep it Java-friendly.

Please make the changes on a new branch from master.