Remove RenderingServer::map_scaling_option_to_stretch_mode and cleanup boot splash code

Removes RS as a dependency in `project_settings.cpp` (which was a bug,
`core` shouldn't include `servers`). This doesn't have a big impact on
incremental rebuild time by itself.

Also move helper `get_splash_stretched_screen_rect` to RenderingServerTypes.

servers/rendering/renderer_compositor.cpp

@@ -30,9 +30,8 @@
 
 #include "renderer_compositor.h"
 
-#include "core/config/project_settings.h"
-
 #ifndef XR_DISABLED
+#include "core/config/project_settings.h"
 #include "servers/xr/xr_server.h"
 #endif // XR_DISABLED
 
@@ -45,11 +44,6 @@ RendererCompositor *RendererCompositor::create() {
 	return _create_func();
 }
 
-void RendererCompositor::set_boot_image(const Ref<Image> &p_image, const Color &p_color, bool p_scale, bool p_use_filter) {
-	RSE::SplashStretchMode stretch_mode = RenderingServer::map_scaling_option_to_stretch_mode(p_scale);
-	set_boot_image_with_stretch(p_image, p_color, stretch_mode, p_use_filter);
-}
-
 bool RendererCompositor::is_xr_enabled() const {
 	return xr_enabled;
 }