This services output capture tasks for the 'framebuffer' and 'blending'
pixel sources.

Just like the old screenshooting path, the 'framebuffer' pixel source is
the hardware buffer, whether a shadow is used or not. This may not be
the best for performance, but you do get the real framebuffer contents.
Maybe it's rgb565, or even less.

When the shadow buffer is used, I realized it is effectively the same as
the intermediate blending buffer in GL-renderer when color management is
used. Pixman-renderer does non-linear blending only, so the shadow
buffer is in the blending space. The shadow buffer is also always 8 bpc
regardless of the hardware framebuffer, so the read-back may be
different from the hardware framebuffer. Read-back from the shadow is
optimal for performance, but not what the hardware gets.

'full-framebuffer' source cannot yet be implemented, because backends do
not tell Pixman-renderer about the margins where the wayland-backend
blits the output decorations. The target "hardware" buffer handed to
pixman-renderer does not allow accessing the decorations area.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>