TilingScheme: Add support for variable tiles splitting

[gkjohnson]

• Add support for variable tile splitting (variable number of children per child layer)
• Fix DebugTilesPlugin not respecting an initial "enabled" flag

TODO

• Consider how this kind of variable tiling can be specified at the user level
  • Possibly a custom-tiled image source? Or allow more flexible parameters to XYZImageSource?

Required for loading this data set which contains a variable number of splits per tile, stored in child kml tiles:

// modifications made to XYZImageSource
tiling.setProjection( new ProjectionScheme( 'EPSG:4326' ) );
tiling.setContentBounds( ...tiling.projection.getBounds() );
tiling.setLevel( 1, {
	tileCountX: 4,
	tileCountY: 2,
	tileSplitX: 3,
	tileSplitY: 3,
} );

tiling.setLevel( 2, {
	tileCountX: 4 * 3,
	tileCountY: 2 * 3,
	tileSplitX: 3,
	tileSplitY: 3,
} );

tiling.setLevel( 3, {
	tileCountX: 4 * 3 * 3,
	tileCountY: 2 * 3 * 3,
	tileSplitX: 5,
	tileSplitY: 5,
} );

tiling.setLevel( 4, {
	tileCountX: 4 * 3 * 3 * 5,
	tileCountY: 2 * 3 * 3 * 5,
	tileSplitX: 2,
	tileSplitY: 2,
} );

tiling.setLevel( 5, {
	tileCountX: 4 * 3 * 3 * 5 * 2,
	tileCountY: 2 * 3 * 3 * 5 * 2,
	tileSplitX: 2,
	tileSplitY: 2,
} );

tiling.setLevel( 6, {
	tileCountX: 4 * 3 * 3 * 5 * 2 * 2,
	tileCountY: 2 * 3 * 3 * 5 * 2 * 2,
	tileSplitX: 2,
	tileSplitY: 2,
} );