Enable enumeration of writeback connectors if both libdrm and the device
support it. The new Card::has_writeback() method report if the card
support writeback connectors.

Existing code that expect all connectors to model an output may be
confused by the sudden availability of new connectors. To handle this
issue,

- add a KMSXX_DISABLE_WRITEBACK_CONNECTORS environment variable to
  disable enumeration of writeback connectors, similarly to universal
  planes ; and

- ignore writeback connectors where no specific connector is requested
  (Card::get_first_connected_connector(),
  ResourceManager::reserve_connector() if no connector name is
  specified, and applications that use all connected outputs).

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

We can use std::format() and a custom print() wrapper with C++20. When
moving to C++23, we can drop the wrapper.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

gcc 11 and earlier do not support constexpr strings and vectors. Add
tests and ifdefs to only use constexpr on selected places for gcc 12+.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>