Fix build on main branch

Fix build on main branch

This commit fixes several build errors and warnings on the main branch,
as reported by clang/GCC on a Ubuntu 20.04 system. The warnings were
also being converted to errors in check_build_sanity, preventing the
checks from completing successfully.

* VulkanSC did not compile with DEQP_SUPPORT_DRM.
* Some GL callbacks used the wrong prototype.
* Some functions had write-only variables that were not being used.
* One class was not using the "override" suffix for virtual methods
  consistently.

VK-GL-CTS issue: 4032
Components: OpenGL, Vulkan, Framework

Change-Id: I9c3e1af13713f1f6ed10cde1c475f7a1c7e6581a