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
67c91353e0