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

doc/en/content-packs.md

@@ -89,6 +89,7 @@ Don't be intimidated by the following text, as a minimal pack only requires `pac
     - `textures/` - Textures
     - `shaders/` - Shaders
         - `effects/` - Post-processing effects
+        - `materials/` - Block material shaders
     - `sounds/` - Sounds and Music
     - `texts/` - Localization files
 - GUI: