High DPI about sdl12-compat HOT 4 CLOSED

Yeah, but we probably need an environment variable to force it off if someone wants less pixels for more performance.

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FWIW I ran into this in implementing high-DPI support in FNA - the end result was that we defaulted to having it off and using the environment variable to turn it on:

https://github.com/FNA-XNA/FNA/wiki/7:-FNA-Environment-Variables#fna_graphics_enable_highdpi

There were two reasons for this:

• The XNA API did not anticipate high-DPI drawables, similar to SDL 1.2. Also vaguely similar to SDL 1.2 is that the window size is based on the backbuffer size, the latter being what the programmer sets - XNA has PreferredBackbufferWidth/Height, SDL has SetVideoMode. Both have sets and neither have gets (necessarily, you can dig in and get the info but it digs into uncharted territory), and conveniently it doesn't strictly say the video mode is the window size. We ultimately decided that the backbuffer size should get priority over the window size; when FNA HighDPI is on we actually shrink the window so that the video mode technically matches what the program requested, at the expense of the window being small. This sounds weird until you consider that A: Many games are picky about the viewport size and would actually break if we made the backbuffer bigger than what they expected, and B: Consider the fullscreen scenario, where the game will request full 4K but the desktop's simulated size will be 1920x1080, so while the game thinks it's running a huge window it's actually just conforming to exactly how high-DPI works, with no weird application hacks required!
• For low-res games, defaulting to high-DPI is actually a problem! A good test case for this is VVVVVV; right now it defaults to always trying high-DPI, but the problem is that the default backbuffer size is 320x240, and since we prioritize backbuffer size over window size...

It's a default setting for ants, but also a very likely default for old games using SDL 1.2! I've sort of looked the other way on this in VVV because I want to make fullscreen the default, which will make this unnoticeable 99% of the time, but it's still really funny when you first load it and a postage stamp suddenly pops up on your monitor.

This is probably way more text than was necessary, but I've worked on supporting this a lot so this is probably the exact one thing that I can actually contribute to 12-compat beyond just throwing old games at it. For a live example, load up the FEZ Steam beta for macOS or Wayland and enable the high-DPI mode in the video options menu.

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I stuck a thing in there (defaulting to OFF). Good enough, everyone?

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lgtm!

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