[wimp] Turn on multithreading for wimp. (#191747)

This turns multithreaded wimp on. This fixes a few issues that popped up
when diagnosing problems related to multithreading:
* Instead of directly transferring `VideoFrame` objects to the web
worker, make an `ImageBitmap` out of it. This normalizes the EXIF
rotation of the image so that we don't end up with problems
transferrring it to a texture.
* Use globals for the raster thread rather than function-local statics.
Emscripten uses some complex locking schemes for function-local statics
that can sometimes cause compilcations and deadlocks, and they are
overkill for what we're doing here. Using globals simplifies any locking
schemes.
* Pass the `ContentContext` when generating the impeller texture. This
makes sure that paths like the picture-to-image code path can share
resources like the typographer context and cached runtime effects.
* Because we're sharing the `ContentContext` when rendering pictures to
images now, we do a pre-pass that renders pictures to their image
representations before starting the rendering of the frame and puts them
in the image cache. This avoids having nested render passes inside of a
frame, which can cause problems for the state of the glyph atlas in the
`ContentContext`