Normal maps generation on the fly.
Parallax mapping with slope information.
Overriding normal maps.
Fix serializing of signed numbers in serializeStructToString
Merge remote-tracking branch 'minetest/master'

Conflicts:
	builtin/mainmenu.lua
	client/shaders/alpha_shader/opengl_fragment.glsl
	client/shaders/leaves_shader/opengl_fragment.glsl
	client/shaders/liquids_shader/opengl_fragment.glsl
	client/shaders/liquids_shader/opengl_vertex.glsl
	client/shaders/plants_shader/opengl_fragment.glsl
	client/shaders/solids_shader/opengl_fragment.glsl