Pekka Paalanen authored Nov 21, 2018 and Jonas Ådahl committed Jan 30, 2019

When setting up an output on a secondary GPU with the CPU copy mode,
allocate the dumb buffers with a DRM format that is advertised supported
instead of hardcoding a format.

Particularly, DisplayLink devices do not quite yet support the hardcoded
DRM_FORMAT_XBGR8888. The proprietary driver stack actually ignores the
format assuming it is DRM_FORMAT_XRGB8888 which results the display
having red and blue channels swapped. This patch fixes the color swap
right now, while taking advantage if the driver adds support for XBGR
later.

The preferred_formats ordering is somewhat arbitrary. Here it is written
from glReadPixels point of view, based on my benchmarks on Intel Haswell
Desktop machine. This ordering prefers the format that was hardcoded
before.

GNOME/mutter!341