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

(cherry picked from commit 67c91353e0)

include/SDL_video.h

@@ -1342,7 +1342,7 @@ extern DECLSPEC int SDLCALL SDL_UpdateWindowSurface(SDL_Window * window);
  *
  * \param window the window to update
  * \param rects an array of SDL_Rect structures representing areas of the
- *              surface to copy
+ *              surface to copy, in pixels
  * \param numrects the number of rectangles
  * \returns 0 on success or a negative error code on failure; call
  *          SDL_GetError() for more information.