Setting the `transform` attribute of the collection **dramatically** slows down rendering

[andrewdnolan]

Commenting out these lines results in a huge speed up:

mosaic/mosaic/polypcolor.py

Lines 70 to 72 in a3af718

	# only set the transform if GeoAxes
	if isinstance(ax, GeoAxes):
	collection.set_transform(transform)

Using the RRSwISC6to18E3r5 mesh and the HEAD of main:

$ time ./src/ocean_hires.py

real    3m27.891s
user    3m31.602s
sys     0m8.719s

but when I comment out the lines of interest:

$ time ./src/ocean_hires.py

real    1m4.880s
user    1m9.175s
sys     0m7.143s

The figures generated are not different in any discernible way (to me). Since we do the coordinate data transformation ourselves and fix the periodicity ourselves (in the descriptor), so I'm not sure we need to set the transform. I'm going to open an issue in cartopy to figure out if we really need to setting the transform.