### 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
if (quiet_stdout) {
		CoreGlobals::print_line_enabled = false;
	}
	Logger::set_flush_stdout_on_print(GLOBAL_GET("application/run/flush_stdout_on_print"));
	OS::get_singleton()->set_cmdline(execpath, main_args, user_args);
	{
		String driver_hints = "";
#ifdef VULKAN_ENABLED
		driver_hints = "vulkan";
#endif
		String default_driver = driver_hints.get_slice(",", 0);
		
		GLOBAL_DEF("rendering/rendering_device/driver", default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/rendering_device/driver.windows", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/rendering_device/driver.linuxbsd", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/rendering_device/driver.android", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/rendering_device/driver.ios", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/rendering_device/driver.macos", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		driver_hints = "";
#ifdef GLES3_ENABLED
		driver_hints += "opengl3";
#endif
		default_driver = driver_hints.get_slice(",", 0);
		GLOBAL_DEF("rendering/gl_compatibility/driver", default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/gl_compatibility/driver.windows", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/gl_compatibility/driver.linuxbsd", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/gl_compatibility/driver.web", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/gl_compatibility/driver.android", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/gl_compatibility/driver.ios", PROPERTY_HINT_ENUM, driver_hints), default_driver);
		GLOBAL_DEF(PropertyInfo(Variant::STRING, "rendering/gl_compatibility/driver.macos", PROPERTY_HINT_ENUM, driver_hints), default_driver);