use terra

[mdsumner]

we'll get away with a CRAN update but wbg to do this properly

• [x] add maxcells, #19
• [x] support SpatRaster in mesh_plot and quadmesh #39
• [ ] refactor mesh_plot to have quadmesh() do all the work via terra (including maxcells)
• [ ] consider not aggregate() but project() if crs is in use
• [ ] fix quadmesh() for texture argument (currently needs raster, not terra)
• [ ] fix mesh_plot() for coords argument (currently needs a brick, not terra)