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/lib/commons.glsl

@@ -8,4 +8,13 @@ vec3 apply_planet_curvature(vec3 modelPos, vec3 pos3d) {
     return modelPos;
 }
 
+// chunk vertex flags (normal.w): bit 7 - emission flag, bits 0-6 - material
+float unpack_emission(float w) {
+    return float(int(w * 255.0 + 0.5) >> 7);
+}
+
+int unpack_material(float w) {
+    return int(w * 255.0 + 0.5) & 127;
+}
+
 #endif // COMMONS_GLSL_

res/shaders/lib/world_fragment_header.glsl

@@ -11,6 +11,7 @@ in vec3 a_realnormal;
 in vec3 a_skyLight;
 in vec4 a_modelpos;
 in float a_emission;
+flat in int a_material;
 
 #include <world_uniforms>
 

res/shaders/lib/world_vertex_header.glsl

@@ -11,6 +11,7 @@ out vec3 a_realnormal;
 out vec3 a_skyLight;
 out vec4 a_modelpos;
 out float a_emission;
+flat out int a_material;
 
 #include <world_uniforms>