TextureDescriptor::theoretical_memory_footprint

[sagudev]

Connections
Inspired by https://bugzilla.mozilla.org/show_bug.cgi?id=2061941

Description
Add TextureDescriptor::theoretical_memory_footprint that estimates memory footprint of a texture. This will be also useful in servo.

Testing
None

Squash or Rebase?
Squash

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.

[teoxoy]

LGTM but @cwfitzgerald should we also use this in the D3D12 backend (ea84239)? Since right now mips are not taken into account.