### 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
}
	OS::get_singleton()->print("].\n");
	OS::get_singleton()->print("  --audio-output-latency <ms>       Override audio output latency in milliseconds (default is 15 ms).\n");
	OS::get_singleton()->print("                                    Lower values make sound playback more reactive but increase CPU usage, and may result in audio cracking if the CPU can't keep up.\n");
	OS::get_singleton()->print("  --rendering-method <renderer>     Renderer name. Requires driver support.\n");
	OS::get_singleton()->print("  --rendering-driver <driver>       Rendering driver (depends on display driver).\n");
	OS::get_singleton()->print("  --gpu-index <device_index>        Use a specific GPU (run with --verbose to get available device list).\n");
	OS::get_singleton()->print("  --text-driver <driver>            Text driver (Fonts, BiDi, shaping).\n");
	OS::get_singleton()->print("  --tablet-driver <driver>          Pen tablet input driver.\n");
	OS::get_singleton()->print("  --headless                        Enable headless mode (--display-driver headless --audio-driver Dummy). Useful for servers and with --script.\n");
	OS::get_singleton()->print("  --write-movie <file>              Writes a video to the specified path (usually with .avi or .png extension).\n");
	OS::get_singleton()->print("                                    --fixed-fps is forced when enabled, but it can be used to change movie FPS.\n");
	OS::get_singleton()->print("                                    --disable-vsync can speed up movie writing but makes interaction more difficult.\n");
	OS::get_singleton()->print("                                    --quit-after can be used to specify the number of frames to write.\n");
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("Display options:\n");
	OS::get_singleton()->print("  -f, --fullscreen                  Request fullscreen mode.\n");
	OS::get_singleton()->print("  -m, --maximized                   Request a maximized window.\n");
	OS::get_singleton()->print("  -w, --windowed                    Request windowed mode.\n");
	OS::get_singleton()->print("  -t, --always-on-top               Request an always-on-top window.\n");
	OS::get_singleton()->print("  --resolution <W>x<H>              Request window resolution.\n");
	OS::get_singleton()->print("  --position <X>,<Y>                Request window position (if set, screen argument is ignored).\n");
	OS::get_singleton()->print("  --screen <N>                      Request window screen.\n");
	OS::get_singleton()->print("  --single-window                   Use a single window (no separate subwindows).\n");
	OS::get_singleton()->print("  --xr-mode <mode>                  Select XR (Extended Reality) mode ['default', 'off', 'on'].\n");
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("Debug options:\n");
	OS::get_singleton()->print("  -d, --debug                       Debug (local stdout debugger).\n");
	OS::get_singleton()->print("  -b, --breakpoints                 Breakpoint list as source::line comma-separated pairs, no spaces (use %%20 instead).\n");
	OS::get_singleton()->print("  --profiling                       Enable profiling in the script debugger.\n");