This allows us to efficiently handle a lot of workloads that require multiple
HTML canvases, without CPU readback. For example, you can render paths to a
render target, then turn that render target into a repeating pattern that you
fill other paths with. Or you can render paths to a render target and then
composite that render target as a whole with a blend mode.
This introduces a new `DrawRenderTarget` render command that blits a render
target without any paths involved. This is basically just a hack that works
around the fact that our tiled renderer doesn't yet support effects that widen
the ink region (i.e. blurs). It can be removed once we have that support.