Added low level 2D API example and some WinAPI window behaviour bug fixes.

Added example of using engine with low level 2D API.
Added event to handle fullscreen mode switch.
Added method to get desktop resolution.
Renamed som render 2D methods to be more readable.
Added method to reset 2D camera.
Close #113
Fixed some erros in 2D rendering.
Added support of 11025 frequency sounds.
Other minor improvments and fixes.