Hotfix/retina screen

[Algiane]

This commit fix (at least partially) issue #6 :

• default behaviour of Medit is unchanged;
• the new -r command line option allow to enable hdpi retina screen support (we doble pixel sizes if needed by the openGL function).

I have tested:

• picking;
• clipping;
• window redimensionning
• splitview mode
• animate mode
• mesh saving using hardcopy.

Possible improvement: I haven't succeed to automatically compute the pixel density using current Medit dependencies (it is possible using cocoa or glfw libraries) so high pixel density is manually enabled with the -r option.

Known issue :

• passing a window from a retina screen to a non retina one (or invert) don't work (it would work with automatic computation of pixel density).