gl_device: Filter more specifically for slow ASTC

Adds a check to find if the renderer is Intel DG (i.e. DG2).

gl_device: Detect Mesa to disable their ASTC

In our testing, our own ASTC decoder has shown itself to perform faster
than the included one from the driver. Disable theirs when Mesa is
detected.

Mesa detection depends on the vendor string. Some drivers never appear
outside of *nix contexts, so only check those in the *nix context.

gl_device: Internalize Intel DG detection