HttpEngine: throw exception instead of null return in conn.read()

- convert OkHttps FileNotFound to some more useful exception
implement okhttp response caching

based on patch from: Baldur Gudbjornsson <baldur.gudbjornsson@gmail.com>
add TileSource.Builders
android: enable GLSurfaceView debug flags with GLAdapter.debug
android: use confirmed tap gesture

- otherwise tap conflicts with double tap zoom..
refactor: hide backend Adapter handles

- static methods for backend adapters
add missing LineBuilder.fixed(bool)
BoxTree:

- return false from search() when search callback returned false
- fix removal: unlink next item
- add clearToPool()
sed 's/m\(in\|ax\)\(X\|Y\)/\L\2m\1/g'
use text texture size for buffered gdx/awt image loading
increase label Texture size
TextureItem: allow cloning uninitialized textures

- TextureRegion: use texture handle, not atlas.texture
- update SymbolLayer

- TextureAtlas: remove depth