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>

Add a new method to write the contents of a framebuffer to a file
descriptor. This can be used to capture frames from writeback
connectors.

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

Finding an LTO capable linker seems to fail for clang.

Replace the hand-rolled draw_color_bar call in FlipState::draw_bar with
pixpat's "vbar" pattern via a new draw_vbar_pattern helper in kms++util.
The pattern fills the full buffer, so the old_xpos bookkeeping for
clearing the previous bar is no longer needed.

Switch draw_test_pattern() to call libpixpat (linked statically into
libkms++util.so) instead of the in-tree pattern generator. Pixpat
covers every pattern (kmstest, smpte, solid colors) and every pixel
format the previous generator handled, so behavior is unchanged for
all callers.

Drop the now-unused machinery: conv.h and conv-*.h template writers,
color16.h (RGB16/YUV16 plus conversions), the *_old / _single / _multi
declarations that had no definitions or callers, and the c_draw_test_pattern
C ABI which had no callers anywhere in the tree. RecStandard and
ColorRange move from color16.h directly into kms++util.h, since they
are still part of the public TestPatternOptions struct.

git-subtree-dir: subprojects/pixpat
git-subtree-split: d444626e6ba988ec6d487800721e447f94b1eaf5