add block material shaders

- 'shader' property of block materials: shaders/materials/<name>.glslv, .glslf with material_vertex / material_fragment hooks and #param uniforms
- material shaders code is included into main, translucent and shadows programs through generated headers, no extra draw calls
- chunk vertex normal.w byte replaced with 'flags': bit 7 - emission flag, bits 0-6 - material shader index (vertex format size is not changed)
- doc/*/block-materials.md

res/shaders/clouds.glslv

@@ -41,7 +41,7 @@ void main() {
     a_fog = calc_fog(length(viewmodel * vec4(pos3d * FOG_POS_SCALE, 0.0)) / 256.0);
 #endif
 
-    a_emission = v_normal.w;
+    a_emission = unpack_emission(v_normal.w);
 
     vec4 viewmodelpos = u_view * a_modelpos;
     a_position = viewmodelpos.xyz;