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/translucent.glslf

@@ -14,8 +14,12 @@ uniform bool u_debugNormals;
 
 #include <shadows>
 
+#include <__materials_fragment__>
+
 void main() {
-    vec4 texColor = texture(u_texture0, a_texCoord);
+    vec4 texColor = apply_material_fragment(
+        a_material, texture(u_texture0, a_texCoord)
+    );
     float alpha = texColor.a;
     if (u_alphaClip) {
         if (alpha < 0.2f)