This patch fixes an issue where the splash screen width clamping
calculation would result in a very narrow size on HiDPI/Retina
display due to the native pixel size not being taken into account.
Fixed by using the `WM_window_native_pixel_x()` function to obtain
the horizontal window size instead of directly using
`CTX_wm_window(C)->sizex`.
Pull Request: https://projects.blender.org/blender/blender/pulls/137997
90b735c28c