Building on the great inital gfx_api and Vulkan work by @vlj, we’re now testing multiple new graphics backend support (slated for release in 3.4.0):

• OpenGL ES 3.0 / 2.0
• Vulkan 1.0+
• DirectX (via LibANGLE) (on Windows)

This is in addition to the existing support for:

• OpenGL 3.0+ Core Profile (default), OpenGL 2.1 Compatibility Profile

On macOS, Vulkan is supported via MoltenVK, which means Warzone2100 is capable of running on top of Metal now as well.

This PR also adds many other enhancements, including:

• A new “Adaptive Vsync” option (where supported), in addition to Vsync on / off
• A new “Graphics Backend” option in the Video Options menu
• Includes “Allow optional per-vertex normals in PIE format” #338
• Substantial improvements to resources cleanup
• and more

For more details, and to obtain a test build, see the PR: https://github.com/Warzone2100/warzone2100/pull/489

This work is currently slated to become part of 3.4.0.