Remove PNG and ZLIB support from the project

- Eliminated all references to PNG and ZLIB dependencies in CMake configuration and project files.
- Updated documentation to reflect the removal of PNG support and related functionalities.
- Adjusted build scripts and source files to remove unused code related to PNG handling, ensuring a cleaner codebase.

scripts/agent.sh

@@ -103,7 +103,7 @@ ubuntu_bootstrap() {
   if command -v apt-get >/dev/null 2>&1; then
     sudo apt-get update
     sudo apt-get install -y inotify-tools build-essential cmake ninja-build pkg-config \
-      libsdl2-dev libpng-dev libglew-dev libwavpack-dev libabsl-dev \
+      libsdl2-dev libglew-dev libwavpack-dev libabsl-dev \
       libboost-all-dev libboost-python-dev python3-dev libpython3-dev
   else
     echo "apt-get not found; skipping Ubuntu bootstrap" >&2