testvulkan: Clamp the drawable size to the allowed range SDL_Vulkan_GetDrawableSize() doesn't always return a size which is within the Vulkan swapchain's allowed image extent range. (This happens on X11 a lot when resizing, which is bug #3287) Clamp the value we get back from SDL_Vulkan_GetDrawableSize() to this range. Given the range usually is just a single value, this is almost always equivalent to just using the min or max image extent, but this seems logically most correct.