Add function to clear bindgroups from passes.

Since the spec allows null BindGroups to be passed to setBindGroup, we
need a way to clear the bindGroup associated with an index/slot.

wgpu-core/src/command/bundle.rs

@@ -1568,6 +1568,14 @@ pub mod bundle_ffi {
         });
     }
 
+    #[no_mangle]
+    pub extern "C" fn wgpu_render_bundle_clear_bind_group(
+        bundle: &mut RenderBundleEncoder,
+        index: u32,
+    ) {
+        bundle.current_bind_groups.clear_bind_group(index)
+    }
+
     #[no_mangle]
     pub extern "C" fn wgpu_render_bundle_set_pipeline(
         bundle: &mut RenderBundleEncoder,

wgpu-core/src/command/compute.rs

@@ -985,6 +985,14 @@ impl Global {
         Ok(())
     }
 
+    pub fn wgpu_compute_pass_clear_bind_group<A: HalApi>(
+        &self,
+        pass: &mut ComputePass<A>,
+        index: u32,
+    ) {
+        pass.current_bind_groups.clear_bind_group(index)
+    }
+
     pub fn compute_pass_set_pipeline<A: HalApi>(
         &self,
         pass: &mut ComputePass<A>,

wgpu-core/src/command/mod.rs

@@ -861,6 +861,13 @@ impl BindGroupStateChange {
         }
         false
     }
+
+    fn clear_bind_group(&mut self, index: u32) {
+        if let Some(current_bind_group) = self.last_states.get_mut(index as usize) {
+            current_bind_group.reset();
+        }
+    }
+
     fn reset(&mut self) {
         self.last_states = [StateChange::new(); hal::MAX_BIND_GROUPS];
     }

wgpu-core/src/command/render.rs

@@ -2488,6 +2488,10 @@ pub mod render_commands {
         });
     }
 
+    pub fn wgpu_render_pass_clear_bind_group(pass: &mut RenderPass, index: u32) {
+        pass.current_bind_groups.clear_bind_group(index)
+    }
+
     pub fn wgpu_render_pass_set_pipeline(pass: &mut RenderPass, pipeline_id: id::RenderPipelineId) {
         if pass.current_pipeline.set_and_check_redundant(pipeline_id) {
             return;