[wsi] For SDL3 use GetWindowSizeInPixels

[flibitijibibo]

Another subtle thing in SDL3 is that "points" vs "pixels" became very separated; you could calculate this manually by using Points and getting the pixel density of the display, but this is a lot easier. (Interestingly this feature is not available for the Set function, so some people are still doing some scaling manually...)

Fixes Wayland windows getting a partial view of the swapchain instead of the full surface; X was not affected.

[doitsujin]

Is this actually the correct thing to do for windowed mode swapchains? DXGI has no real notion of desktop vs display coordinates, and Windows seems to do scaling at a compositor level rather than giving the app a larger swapchain, but I'm not 100% sure what the exact behaviour is there.

For full-screen i guess the issue is that the window's desktop coordinates are smaller than the display mode.

[doitsujin]

There's also the question of what to do with DXGI ResizeTarget. Not sure if that's supposed to be display or desktop coordinates if the app is high dpi aware (which, again, we don't really have a notion of, but Windows does).

[flibitijibibo]

At least from SDL's point of view this should be more correct; the calls line up with dxvk's win32 WSI:

https://github.com/doitsujin/dxvk/blob/master/src/wsi/win32/wsi_window_win32.cpp#L109

https://github.com/libsdl-org/SDL/blob/main/src/video/windows/SDL_windowswindow.c#L1044

SDL should probably do the left/top subtraction too... A bonus: Here's where SDL3 does the scaling for Set, which results in GetWindowSize being different on Windows too: https://github.com/libsdl-org/SDL/blob/main/src/video/windows/SDL_windowswindow.c#L264