Implement buffer_size overrides

[nyurik]

• implements https://github.com/openmaptiles/openmaptiles/issues/1345
• partially implements https://github.com/openmaptiles/openmaptiles-tools/issues/377

Layer file changes

• Added support for an optional min_buffer_size value, which is only used in case of a global override, but if set, it must be less than or equal to buffer_size.
• Layer's buffer_size is now optional if min_buffer_size is set.

layer:
  id: housenumber
  # Only one of these is mandatory
  min_buffer_size: 1
  buffer_size: 8

Tileset file changes

• Added support for an optional overrides section (similar to defaults).
  • Only supports optional overrides.buffer_size int value for now. If set, this value will override the buffer_size set in the layer. If layer has min_buffer_size, the largest of the two will be used.
• Added support for per layer overrides as described in https://github.com/openmaptiles/openmaptiles-tools/issues/377. Only supports buffer_size and min_buffer_size
• An environment variable TILE_BUFFER_SIZE can be used instead of overrides.buffer_size, and it takes precedence.

This example will load two layers. Mountain peak will be used as is, and house numbers's min buffer zoom is modified. The global override=0 will set mountain peaks buffer to 0 (because it has no min_buffer_size defined inside the layer file), but the house number buffer size will be 5.

tileset:
  layers:
    - mountain_peak/mountain_peak.yaml
    - file: housenumber/housenumber.yaml
      min_buffer_size: 5
  overrides:
    buffer_size: 0

Breaking

• Remove deprecated tileset and layer properties (they have been deprecated for many years, and are not used)

[farfromrefug]

@nyurik this look pretty awesome!