ioemu: SDL rendering using OpenGL

ioemu: SDL rendering using OpenGL

Add opengl support for rendering the guest framebuffer in the SDL
window. SDL is needed anyway to open the window and handle the
events. Opengl rendering is optional and can be turned off at both
compile time and in the vm configuration file.

Some of the benefits of using opengl are:

 - faster rendering, less CPU intensive, especially with good graphic
   cards;
 - makes the window resizing possible and hardware accelerated, thus
   very efficient and smooth;
 - allows other optimizations like sharing directly a buffer in vram
   with the guest (not yet implemented).

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>