Add better HDPI support

[rmarquis]

Currently, on windows using HDPI and font scaling the game windows is also scaled, making its content blurry. This can be disabled by right clicking on the exe and explicitly disabling HDPI in the compatibility tab.

Fixing this on windows automatically requires some platform dependent call: SetProcessDpiAwareness(PROCESS_PER_MONITOR_DPI_AWARE).

See:

• nlguillemot Blog post - High DPI Rendering
• PROCESS_DPI_AWARENESS enumeration
• SetProcessDpiAwarenessContext function
• High DPI Desktop Application Development on Windows

There is a work-in-progress patch in SDL, that should make it easier/straightforward to disable HDPI stretching without any platform dependent API call, see:

• https://discourse.libsdl.org/t/sdl-window-allow-highdpi-does-not-seem-to-work/26901/2
• New High DPI Support API is needed libsdl-org/SDL#2119

This isn't available in the latest stable version of SDL yet.

Status on other platforms - especially Linux, is unknow.
For macOS, see

• https://wiki.libsdl.org/SDL_HINT_VIDEO_HIGHDPI_DISABLED

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Until SDL handles this, as a temporary workaround, we can

• implement all this code from ETe: https://github.com/etfdevs/ETe/blob/master/src/win32/win_dpi.c, or
• add a registry key from the windows installer to force HighDPI override:

[rmarquis]

As of now, SDL still doesn't support HDPI scaling natively (see libsdl-org/SDL#2119), but some code example of workaround can be found here:

• https://github.com/dhewm/dhewm3/blob/5dd826ee388bf3d6d6331c73558616fdd70b5fb6/neo/sys/win32/win_main.cpp#L764-L805
• flyinghead/flycast@21ab111