Metal: Implement texture_create_from_extension

[kroketio]

This PR implements texture_create_from_extension() for Metal.

The Vulkan device driver implements texture_create_from_extension() to handle externally backed textures (OpenXR module, or otherwise), which takes an incoming VkImage, creates a view, and returns it as TextureID.

godot/drivers/vulkan/rendering_device_driver_vulkan.cpp

Lines 1745 to 1750 in e4e024a

	RDD::TextureID RenderingDeviceDriverVulkan::texture_create_from_extension(uint64_t p_native_texture, TextureType p_type, DataFormat p_format, uint32_t p_array_layers, bool p_depth_stencil) {
	VkImage vk_image = (VkImage)p_native_texture;
	// We only need to create a view into the already existing natively-provided texture.
	VkImageViewCreateInfo image_view_create_info = {};

For Metal, there is no exact equivalent to Vulkan's VkImageView, so for this driver we only need to cast the incoming handle to id<MTLTexture>, then return as TextureID.

Parameters p_type, p_format, p_array_layers, p_depth_stencil are ignored - MTLTexture (and the callee) already have this information and is only relevant when reinterpreting or remaping the texture in different ways.

The use of __bridge with void* in the casting is to signal we are not transferring ownership, and is a safe cast to bridge from C-style memory to Objective-C memory management (but correct me if I'm wrong).

loosely related to #96860

[dsnopek]

I don't know much about Metal, in general, but lately I've been spending a bunch of time in this part of the RenderingDevice and RenderingServer API, and this seems about right to me.

However, it would be good for @stuartcarnie and the rendering team to take a look

[stuartcarnie]

This may not be relevant here, as the client can create the new view over the texture themselves. In Metal there is a the ability to create a new texture view for an existing id<MTLTexture>:

- (id<MTLTexture])newTextureViewWithPixelFormat:(MTLPixelFormat)pixelFormat 
                                    textureType:(MTLTextureType)textureType 
                                         levels:(NSRange)levelRange 
                                         slices:(NSRange)sliceRange 
                                        swizzle:(MTLTextureSwizzleChannels)swizzle;

[kroketio]

This is the reason why this PR does not add a view but simply casts and returns, as opposed to the Vulkan implementation where it adds a view. For context, I have a foreign MTLTexture (from a different rendering engine) and to get that texture usable in Godot, is to go through RenderingDevice::texture_create_from_extension() which for Metal will end up calling RenderingDeviceDriverMetal::texture_create_from_extension() - which is expected to return TextureID.

[akien-mga]

Thanks!