glamoregl: Initialize glamor on the main device

This allows e.g. an UDL device (driven by llvmpipe) to be automatically
set up with GPU acceleration via reverse PRIME.

The result is e.g.:

  # DISPLAY=:0.2 xrandr --listproviders
  Providers: number : 2
  Provider 0: id: 0xec cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 1 outputs: 1 associated providers: 1 name:modesetting
  Provider 1: id: 0x12c cap: 0xb, Source Output, Sink Output, Sink Offload crtcs: 2 outputs: 2 associated providers: 1 name:Intel

Signed-off-by: Zoltán Böszörményi <zboszor@gmail.com>
13 jobs for fix-gpu-screen-assignment in 4 minutes and 23 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed debian-buster #12334376

00:00:10

 
  Build And Test
passed autotools #12334385

00:04:12

passed meson #12334386

00:01:34

passed meson-noglamor #12334387

00:02:03

passed mingw-cross-build #12334388

00:00:44

failed autotools #12334377

00:00:16

failed autotools #12334381

00:00:15

failed meson #12334378

00:00:16

failed meson #12334383

00:00:15

failed meson-noglamor #12334379

00:00:16

failed meson-noglamor #12334382

00:00:16

failed mingw-cross-build #12334384

00:00:15

failed mingw-cross-build #12334380

00:00:16