Codechange: [SDL2/Win32] Be consistent how 0bpp blitters are not allowed

Sometimes it returned an usererror(), sometimes Start() failed.
Now it always fails on Start(), so nothing else has to check again
what blitter is used.

AfterBlitterChange() can never change to a 0bpp, so it is sufficient
to guard this with an assert().

src/video/sdl2_v.cpp

@@ -258,8 +258,6 @@ bool VideoDriver_SDL::CreateMainSurface(uint w, uint h, bool resize)
 
 	DEBUG(driver, 1, "SDL2: using mode %ux%ux%d", w, h, bpp);
 
-	if (bpp == 0) usererror("Can't use a blitter that blits 0 bpp for normal visuals");
-
 	/* Free any previously allocated shadow surface */
 	if (_sdl_surface != nullptr && _sdl_surface != _sdl_realscreen) SDL_FreeSurface(_sdl_surface);
 
@@ -664,6 +662,8 @@ int VideoDriver_SDL::PollEvent()
 
 const char *VideoDriver_SDL::Start(const StringList &parm)
 {
+	if (BlitterFactory::GetCurrentBlitter()->GetScreenDepth() == 0) return "Only real blitters supported";
+
 	/* Explicitly disable hardware acceleration. Enabling this causes
 	 * UpdateWindowSurface() to update the window's texture instead of
 	 * its surface. */
@@ -939,6 +939,7 @@ bool VideoDriver_SDL::ToggleFullscreen(bool fullscreen)
 
 bool VideoDriver_SDL::AfterBlitterChange()
 {
+	assert(BlitterFactory::GetCurrentBlitter()->GetScreenDepth() != 0);
 	int w, h;
 	SDL_GetWindowSize(_sdl_window, &w, &h);
 	return CreateMainSurface(w, h, false);