### Godot version

4.1

### System information

Windows 10 - Godot 4.1.1 - Compatibility

### Issue description

When trying to use the Varjo XR3 headset (in OpenXR mode) with Godot 4 (with OpenXR and passthrough activated), the passthrough does not work, i.e. the scene is rendered on a black background instead of the camera stream.

It seems to me that are two problems in the openxr module, more precisely in the openx_api.cpp file (fixing them solves  the issue) : 

1. The XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT  is only set when the number of layers is > 1, which is fine for the Meta Quest headset but does not work for the Varjo since it only requires one layer. 
2. The environment blend mode XR_ENVIRONMENT_BLEND_MODE_ALPHA_BLEND  is only really set when the XRFbPassthrough extension is present, which is not the case with the Varjo runtime. 

I am not sure how to fix this without causing issues for other headsets.

1. Would it be an issue to always activate the XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT ? 
2. Could blend modes be activated even if they are not supported by an extension ? 





### Steps to reproduce

- Create a Godot 4 project with OpenXR activated in Compatibility mode
- Add the code to activate OpenXr and to enable passthrough : https://docs.godotengine.org/en/stable/tutorials/xr/openxr_passthrough.html#passthrough-extension
- Start the Varjo Base software and activate OpenXR
- Run the Godot project

### Minimal reproduction project

N/A
static int text_driver_idx = -1;
static int display_driver_idx = -1;
static int audio_driver_idx = -1;
static bool single_window = false;
static bool editor = false;
static bool project_manager = false;
static bool cmdline_tool = false;
static String locale;
static bool show_help = false;
static uint64_t quit_after = 0;
static OS::ProcessID editor_pid = 0;
#ifdef TOOLS_ENABLED
static bool found_project = false;
static bool auto_build_solutions = false;
static String debug_server_uri;
#ifndef DISABLE_DEPRECATED
static int converter_max_kb_file = 4 * 1024; 
static int converter_max_line_length = 100000;
#endif 
HashMap<Main::CLIScope, Vector<String>> forwardable_cli_arguments;
#endif
static bool single_threaded_scene = false;
static DisplayServer::WindowMode window_mode = DisplayServer::WINDOW_MODE_WINDOWED;
static DisplayServer::ScreenOrientation window_orientation = DisplayServer::SCREEN_LANDSCAPE;
static DisplayServer::VSyncMode window_vsync_mode = DisplayServer::VSYNC_ENABLED;
static uint32_t window_flags = 0;
static Size2i window_size = Size2i(1152, 648);
static int init_screen = DisplayServer::SCREEN_PRIMARY;
static bool init_fullscreen = false;
static bool init_maximized = false;