Bug in 4k60Hz Display EDID

When creating a virtual monitor with 4k@60Hz, the virtual monitor gets a refresh rate of 53.86. This can cause some stuttering in games due to the mismatch of refresh rates.

Fix: In virtual_display.cpp, change the pixel_clock_khz for the 4k 60 Hz EDID from:
pixel_clock_khz = 533250;

to:
pixel_clock_khz = 594000;