Add HTMLImageElement/ImageData as external source for copying images

[valaphee]

Connections
Solves #4236
Depends on grovesNL/glow#291

Description
This adds the missing external sources for HTMLImageElement and ImageData which are useful for removing the need to ship an image decoder with an wasm application.

Testing
TODO: We should add a texture example, which on wasm loads the texture via external image source.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[valaphee]

Now just VideoFrame is missing, but this also seems to be experimental