Fix dask TriMesh rasterization

[philippjfr]

Previously the vertex and simplex data was always converted to pandas so the dask codepath in datashader was never exercised.

[jlstevens]

Looks good to me!

Seems conda on python 3.6 windows is running out of memory:

Solving environment: ...working... failed with initial frozen solve. Retrying with flexible solve.

Solving environment: ...working... failed with repodata from current_repodata.json, will retry with next repodata source.

Collecting package metadata (repodata.json): ...working... done

*** picosat: out of memory in 'resize'
Solving environment: ...working... TaskError - taskid:develop_install
Command error: 'conda install -y  -c pyviz/label/dev -c bokeh/label/dev -c conda-forge "param >=1.9.3,<2.0" "numpy >=1.0" "pyviz_comms >=0.7.4" "panel >=0.8.0" "colorcet" "pandas >=0.20.0" "nose" "mock" "flake8" "coveralls" "path.py" "matplotlib >=3" "nbsmoke >=0.2.0" "nbconvert <6" "twine" "rfc3986" "keyring" "ipython >=5.4.0" "notebook" "matplotlib >=3" "bokeh >=1.1.0" "networkx" "pillow" "xarray >=0.10.4" "plotly >=4.0" "dash >=1.16" "streamz >=0.5.0" "datashader >=0.11.1" "ffmpeg" "cftime" "netcdf4" "dask" "scipy" "shapely" "scikit-image" "spatialpandas" "pyarrow" "ibis-framework >=1.3" "nose" "mock" "flake8" "cov

And coveralls will be fixed by adding a unit test.

[philippjfr]

Ready to merge.