Currently, the SDL_WINDOWEVENT_RESIZED event is sent before the actual
window is resized (and various internal state, such as the desired
GL/Vulkan backbuffer size, are updated). This makes sense, as SDL will
discard a no-op resize, which would be the case if we had resized before
sending the event (indeed, there are existing hacks to prevent this).
However, this means that SDL_{GL,Vulkan}_GetDrawableSize() will still
use the old size in the SDL_WINDOWEVENT_RESIZED handler. In the case of
SDL_Renderer, this means the drawable size it uses will be wrong, and
the viewport will get "updated" to the old value.
This then results in bug #5899.
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