Handle DPI scaling in SDL_GetWindowSurface

Fixes DPI awareness of testdrawchessboard (previously, the surface was
being created in points instead of pixels, resulting in the demo app
only drawing in a corner of the screen on High-DPI displays)

*_CreateWindowFramebuffer()/*_UpdateWindowFramebuffer(): are updated
to use SDL_GetWindowSizeInPixels instead of SDL_GetWindowSize() or
window->w/window->h.

Most of the _CreateWindowFramebuffer backends are untested except
for Windows.

Fixes #7047

src/render/software/SDL_render_sw.c

@@ -90,7 +90,7 @@ static int SW_GetOutputSize(SDL_Renderer *renderer, int *w, int *h)
     }
 
     if (renderer->window) {
-        SDL_GetWindowSize(renderer->window, w, h);
+        SDL_GetWindowSizeInPixels(renderer->window, w, h);
         return 0;
     }