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/shadows.glslf

@@ -1,9 +1,13 @@
-in vec2 a_texCoord;
+#include <world_fragment_header>
 
 uniform sampler2D u_texture0;
 
+#include <__materials_fragment__>
+
 void main() {
-    vec4 tex_color = texture(u_texture0, a_texCoord);
+    vec4 tex_color = apply_material_fragment(
+        a_material, texture(u_texture0, a_texCoord)
+    );
     if (tex_color.a < 0.5) {
         discard;
     }