add fullscreen block overlay

src/voxels/Block.cpp

@@ -141,6 +141,7 @@ void Block::cloneTo(Block& dst) {
     dst.uiLayout = uiLayout;
     dst.inventorySize = inventorySize;
     dst.tickInterval = tickInterval;
+    dst.overlayTexture = overlayTexture;
 }
 
 static std::set<std::string, std::less<>> RESERVED_BLOCK_FIELDS {

src/voxels/Block.hpp

@@ -185,6 +185,9 @@ public:
     /// @brief Block will be used instead of this if generated on surface
     std::string surfaceReplacement = name;
 
+    /// @brief Texture will be shown on screen if camera is inside of the block
+    std::string overlayTexture;
+
     /// @brief Default block layout will be used by hud.open_block(...)
     std::string uiLayout = name;