Forbid passing multiview sampler to the custom function in shaders

servers/rendering/renderer_compositor.cpp

@@ -35,6 +35,8 @@
 #include "core/string/print_string.h"
 #include "servers/xr_server.h"
 
+RendererCompositor *RendererCompositor::singleton = nullptr;
+
 RendererCompositor *(*RendererCompositor::_create_func)() = nullptr;
 bool RendererCompositor::low_end = false;
 
@@ -47,6 +49,8 @@ bool RendererCompositor::is_xr_enabled() const {
 }
 
 RendererCompositor::RendererCompositor() {
+	singleton = this;
+
 	if (XRServer::get_xr_mode() == XRServer::XRMODE_DEFAULT) {
 		xr_enabled = GLOBAL_GET("xr/shaders/enabled");
 	} else {