render: Flush draws before updating bitmap texture

This fixes an issue where new draws were drawn over an old GPU buffer.
For instance, consider the following scenario:

    1. draw (GPU)
    2. full fill (CPU)
    3. draw (GPU)

When full fill is executed, the GPU buffer is discarded without a sync,
but we have to make sure we wait for any queued draws in order to
truly discard 1.

Co-authored-by: Nathan Adams <[email protected]>

render/wgpu/src/backend.rs

@@ -687,6 +687,7 @@ impl<T: RenderTarget + 'static> RenderBackend for WgpuRenderBackend<T> {
             depth_or_array_layers: 1,
         };
 
+        self.active_frame.submit_direct(&self.descriptors);
         self.descriptors.queue.write_texture(
             wgpu::ImageCopyTexture {
                 texture: &texture.texture,