Add option to use texture alpha channel

client/shaders/test_shader_3/base.txt

client/shaders/test_shader_3/opengl_fragment.glsl

client/shaders/test_shader_3/opengl_vertex.glsl

doc/lua_api.txt

src/mapblock_mesh.cpp

src/nodedef.cpp

src/scriptapi_content.cpp

src/tile.h