Bug 1435094 - wire up GlyphRasterSpace to nsDisplayTransform. r=kats,mstange

When a transform thinks it's animated we should abandon screen rasterization
and instead favour local rasterization. This produces a more visually
pleasant rendering, as pixel-snapping "wobbles" the text between
frames.

The float scale of GlyphRasterSpace::Local is currently unused, but this
PR tries its best to set it to a reasonable value, based on discussion
with glennw about the intended semantics. We agreed it should specify
the scale *relative* to the parent stacking context, which means it's
just whatever scaling the stacking context's transform applies. It's
possible we'll need to clamp this value or make it properly 2-dimensional
later on.

Some book-keeping is added to StackingContextHelper to ensure that
GlyphRasterSpace::Screen is never requested by a descendent
of a stacking context using GlyphRasterSpace::Local.

nsDisplayMask is changed to use a StackingContextHelper to ensure
rasterSpace is properly propagated.

In addition, this is the first commit making use of cbindgen's new support
for bridging Rust enums natively into C++! This bumps our minimum cbindgen
to 6.0.0 (just released).

MozReview-Commit-ID: 9AlsB6nUheB

gfx/layers/wr/AsyncImagePipelineManager.cpp

@@ -327,7 +327,9 @@ AsyncImagePipelineManager::ApplyAsyncImages()
                                 nullptr,
                                 pipeline->mMixBlendMode,
                                 nsTArray<wr::WrFilterOp>(),
-                                true);
+                                true,
+                                // This is fine to do unconditionally because we only push images here.
+                                wr::GlyphRasterSpace::Screen());
 
     LayoutDeviceRect rect(0, 0, pipeline->mCurrentTexture->GetSize().width, pipeline->mCurrentTexture->GetSize().height);
     if (pipeline->mScaleToSize.isSome()) {