Suppress two clippy false-positives which are only valid when some features are disabled

[rhysd]

This PR suppresses two false-positives from clippy.

• clippy complains about not using #[default] for Theme enum but it is only valid when auto-detect-theme feature is disabled.

  warning: this `impl` can be derived
     --> core/src/theme.rs:165:1
      |
  165 | / impl Default for Theme {
  166 | |     fn default() -> Self {
  167 | |         #[cfg(feature = "auto-detect-theme")]
  168 | |         {
  ...   |
  187 | |     }
  188 | | }
      | |_^
  

• clippy complains about constructing a unit tuple with default() constructor but it is only valid when both image and svg features are disabled.

  warning: use of `default` to create a unit struct
     --> wgpu/src/layer.rs:290:33
      |
  290 |             images: image::Batch::default(),
      |                                 ^^^^^^^^^^^ help: remove this call to `default`
      |