UI: Correct Splash RGB Image Corner Rounding

Our splash screen rounds the top corners of the featured image. However
this requires an alpha channel, so an RGB image will not look rounded.
This PR just ensures a 32 plane count.

Pull Request: https://projects.blender.org/blender/blender/pulls/119473

source/blender/windowmanager/intern/wm_splash_screen.cc

@@ -158,6 +158,7 @@ static ImBuf *wm_block_splash_image(int width, int *r_height)
   }
 
   if (ibuf) {
+    ibuf->planes = 32; /* The image might not have an alpha channel. */
     height = (width * ibuf->y) / ibuf->x;
     if (width != ibuf->x || height != ibuf->y) {
       IMB_scaleImBuf(ibuf, width, height);