Build fails with recent bevy master

[rparrett]

due to bevyengine/bevy#1236

error[E0046]: not all trait items implemented, missing: `copy_texture_to_buffer`, `copy_texture_to_texture`
  --> /Users/robparrett/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/bevy_webgl2-0.4.2/src/renderer/webgl2_render_context.rs:29:1
   |
29 | impl RenderContext for WebGL2RenderContext {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `copy_texture_to_buffer`, `copy_texture_to_texture` in implementation
   |
   = help: implement the missing item: `fn copy_texture_to_buffer(&mut self, _: TextureId, _: [u32; 3], _: u32, _: BufferId, _: u64, _: u32, _: Extent3d) { todo!() }`
   = help: implement the missing item: `fn copy_texture_to_texture(&mut self, _: TextureId, _: [u32; 3], _: u32, _: TextureId, _: [u32; 3], _: u32, _: Extent3d) { todo!() }`

error: aborting due to previous error