Improve Safari canvas snapshot resizing

[jamilahmadzai]

Summary

• add a WebKit-only snapshot path that pre-scales canvas and video sources before bitmap creation
• keep Chromium and Firefox on the existing createImageBitmap(source, resize*) path
• avoid resize options on WebKit when the source is already at the target size
• add focused coverage for Safari/iOS WebKit gating versus Chromium/Firefox

Validation

• yarn workspace rrweb build
• ./node_modules/.bin/vitest --config rrweb/packages/rrweb/vitest.config.ts run rrweb/packages/rrweb/test/record/canvas-manager.test.ts

Source companion for highlight/highlight#6775.