Cleanup selection mesh code, add shaders for halo and selection boxes

builtin/settingtypes.txt

client/shaders/default_shader/opengl_fragment.glsl

client/shaders/default_shader/opengl_vertex.glsl

client/shaders/selection_shader/opengl_fragment.glsl

client/shaders/selection_shader/opengl_vertex.glsl

minetest.conf.example

src/client.cpp

src/client.h

src/content_mapblock.cpp

src/defaultsettings.cpp

src/drawscene.cpp

src/drawscene.h

src/game.cpp

src/hud.cpp

src/hud.h

src/mapblock_mesh.cpp

src/mapblock_mesh.h

src/mesh.cpp

src/mesh.h

src/nodedef.cpp