Update .gitlab-ci.yml file - ensure that openGL is a thing.

.gitlab-ci.yml

@@ -42,6 +42,7 @@ build:debian:
   stage: build
   before_script:
     - apt update && apt install -y build-essential python3-pip cmake
+    - apt install -y libgl-dev
     - pip install conan
   script:
     - cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug