Vulkan: Fix CanvasItem::use_parent_material

The "Use Parent Material" option now does something when enabled on a CanvasItem. As before, it's not just limited to a node's direct parent but can move up the tree until it finds a material.
Also corrected a typo in rendering_device_vulkan.h that didn't merit its own commit.

drivers/vulkan/rendering_device_vulkan.h

@@ -812,7 +812,7 @@ class RenderingDeviceVulkan : public RenderingDevice {
 	// When using split command lists, this is
 	// implemented internally using secondary command
 	// buffers. As they can be created in threads,
-	// each needs it's own command pool.
+	// each needs its own command pool.
 
 	struct SplitDrawListAllocator {
 		VkCommandPool command_pool = VK_NULL_HANDLE;