The function RRotateImage contains code to handle efficiently the 4 simple
angles 0, 90, 180 and 270 degrees, which makes it a long function.
This patch separate the code for the different cases into dedicated
functions so the main function's code ends up being simple (aka: easier to
understand/review/maintain).
As a side effect, the function for the 180 degree function is not static
because it can be reused to flip an image both horizontally and vertically.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
