Optimize shaders code. Add settings at compile time.

client/shaders/alpha_shader/opengl_fragment.glsl

client/shaders/alpha_shader/opengl_vertex.glsl

client/shaders/leaves_shader/opengl_fragment.glsl

client/shaders/leaves_shader/opengl_vertex.glsl

client/shaders/liquids_shader/opengl_fragment.glsl

client/shaders/liquids_shader/opengl_vertex.glsl

client/shaders/plants_shader/opengl_fragment.glsl

client/shaders/plants_shader/opengl_vertex.glsl

client/shaders/solids_shader/opengl_fragment.glsl

client/shaders/solids_shader/opengl_vertex.glsl

src/game.cpp

src/mapblock_mesh.cpp

src/shader.cpp