Set defaults if MBTile headers omitted during Python PMTile convert

[spwoodcock]

Issue

• While the MBTiles spec says that maxzoom, minzoom, center, bounds should all be included in the metadata table, sometimes they aren't unfortunately.
• We can rely on maxzoom and minzoom, as they are used by underlying tools a fair bit.
• The bounds and center metadata are still useful to underlying tools, but not always used.
• QGIS loads an MBTiles file without these params set, no problem.
• Ideally the conversion shouldn't fail if they aren't all present.

Solution

• I kept the minzoom and maxzoom params as mandatory. If omitted, the code fails attempting to index them.
• I added defaults for:
  • bounds: global bounds
  • center: take the center point of the provided bounds.

[spwoodcock]

I made a plugin to convert mbtiles --> pmtiles, but the conversion fails with pmtiles==3.5.0 due to missing 'center' in the metadata. This MBTiles was generated by QGIS, so it's probably pretty common to have the center metadata missing: