Use real screen size

[AyWa]

fix #20

Description

I am not expert of the macos screen resolution, but we can split:

• the physical size of the screen 2880 x 1800
• the "resolution" / scale -> 1680 x 1050 (can change depending on the scaling settings
• the size of the screen that the system is using before scaling down ( x2 the resolution / scale) -> 3360 x 2100

this patch is using the 3 data -> the size of the screen that the system is using before scaling down -> which is the same behavior that the macos PrtScn is using