Add support for Hi-DPI displays (interface scale factor)

[cschreib]

The whole GUI uses pixels as a base unit. This is fine for "normal" resolution displays, but a given interface will render very tiny on Hi-DPI displays. This can be fixed by applying a global scaling factor to pixel coordinates / dimensions.

Other approaches would include knowing the actual DPI of the display, and automatically adjusting the interface accordingly. This will be less portable, but more user friendly?

[cschreib]

Work for this started in the branch https://github.com/cschreib/lxgui/tree/vbo. The current implementation exposes a global scaling factor that can be tuned (from C++ and Lua). This factor is multiplied to a "hint" provided by the input source (only supported with SDL input), which is effectively the recommended scaling factor value from the operating system.

This means

• By default (with SDL only) the scaling should be right for all platforms.
• The scaling can still be tuned to the user's preference, for example for visually-impaired users who might need larger fonts.

[cschreib]

Merged in master.