Provide Gecko with ImageRendering property when locking NativeTexture external images

[hwinnemoe]

fixes #2993

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This change is 

[hwinnemoe]

Try push of WIP, also includes additional fixes: https://treeherder.mozilla.org/#/jobs?repo=try&revision=4fc42b3e4e77699bff309491005f76ff1d3572fe

[gw3583]

Looks like the examples need to be updated:

error[E0050]: method `lock` has 3 parameters but the declaration in trait `webrender::ExternalImageHandler::lock` has 4
  --> examples\texture_cache_stress.rs:64:62
   |
64 |     fn lock(&mut self, _key: ExternalImageId, channel_index: u8) -> webrender::ExternalImage {
   |                                                              ^^ expected 4 parameters, found 3
   |
   = note: `lock` from trait: `fn(&mut Self, webrender::<unnamed>::ExternalImageId, u8, webrender::<unnamed>::ImageRendering) -> webrender::ExternalImage<'_>`

[gw3583]

The change looks good to me, once the CI errors above are resolved, thanks!

[kvark]

@gw3583 We've got to start tracking the breaking changes more careful as WR gets published.
@bors-servo r=gw3583

[bors-servo]

📌 Commit cd93b95 has been approved by gw3583

[bors-servo]

⌛ Testing commit cd93b95 with merge d88b47c...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: gw3583
Pushing d88b47c to master...