This driver should sunset the old gl2 driver, but that driver will
likely live on to support really ancient and terrible GL stacks.
All the worst legacy cruft has been ripped out, and it's almost a decent
backend now.
Requirements for slang are GL 3.2+ or GLES3.
Some shaders require features which are not directly compatible with
GLES2 or legacy GL.
This driver shares a lot of concepts from the Vulkan driver.
The slang shader stack and SPIRV-Cross are used to implement the shader
spec, and the menu shaders are also shared with Vulkan.
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