[GUI] Support HiDPI / toggle native resolution

[bobcao3]

Concisely describe the proposed feature
Support HiDPI in the GUI or have a toggle to enable native resolution (client side scaling) so the output can be pixel-perfect on fractional scaling displays.

Describe the solution you'd like (if any)
This is relatively simple to implement on Windows and probably non-problem on Linux due to its quirks on X11 scaling & DPI protocols. (todo: attach a commit here) However it may be bad to just use client side scaling and ignoring the DPI, therefore some API changes will be needed to enable DPI aware GUI.

Additional comments
Reference the design of GLFW, SDL, and ImGUI

[bobcao3]

Just checked the behavior of the OpenGL backend, it is using native resolution but it does not seem like the GUI is using the DPI information. Therefore having the WIN32 backend to be pixel perfect should be fine for now.