fix: clear depth buffer for tile clipping

- need to enable depthmask for clear
GeometryBuffer: return self for chaining, add toString()
fix: GeometryBuffer.startLine()

- *always* set end marker
add MapElement.toString()
add LwHttp constructor with additional request header items
LwHttp, ignore readers that try to close InputStream.
improve DebugTheme
add Point utilities
improve LineLayer bevel joint
LineLayer refactor:

- extract addLine()
- expand variable names
LineLayer: drop invalid lines
fix: add default for VectorTileLoader.filterTags()
add VectorTileLayer constructor with TileSource
cleanup: emu VectorTileLayer
theme: only add casing when core is rendered
add getter for UrlTileSource.getUrl()
set default max/min zoom
refactor BitmapTileLayer:

- move to layer.tile.bitmap
- extract BitmapTileLoader
- bring back GWT BitmapTileLayer to life
package vtm-natives with gdx-desktop