Now that each DrawTargetWebgl shares the same WebGL context, we can efficiently draw snapshots
of one DrawTargetWebgl to another without requiring any readback or error-prone driver-provided
shared context/resource mechanism. We just need to simply pass the WebGL texture from one target
to the other, and use it like any other texture.

This provides SourceSurfaceWebgl to store and pass along that WebGL texture. It is largely
modeled off of SourceSurfaceSkia in terms of its copy-on-write behavior. There are three
noteworthy state changes that it must track from DrawTargetWebgl - when the framebuffer
contents is changing, when the framebuffer is being destroyed, and when any cached texture
handle separate from a framebuffer is also being destroyed. It will copy, orphan, or read
back data as appropriate to handle each case.

If it needs to be mapped, it just forces a read back of the data into a CPU surface that
can be mapped as requested.

Differential Revision: https://phabricator.services.mozilla.com/D138118