Add current_used_block_num() in GpuDataRequest to get the current use…

…d block number.

webrender/src/gpu_cache.rs

@@ -468,6 +468,10 @@ impl<'a> GpuDataRequest<'a> {
         self.texture.pending_blocks.extend_from_slice(blocks);
     }
 
+    pub fn current_used_block_num(&self) -> usize {
+        self.texture.pending_blocks.len() - self.start_index
+    }
+
     /// Consume the request and return the number of blocks written
     pub fn close(self) -> usize {
         self.texture.pending_blocks.len() - self.start_index