screen-share: use compositor->read_format with renderer->read_pixels()

The pixel format was hardcoded to PIXMAN_a8r8g8b8. All other
renderer->read_pixels() calls in weston use dynamic format selection via
the compositor->read_format instead.

The problem was spotted on the ARM devices with Mali-400 GPU. The visual
effect of the problem was black screen on the remote display, when using
screen-share with the VNC or RDP backends. Related wayland-devel thread:
https://lists.freedesktop.org/archives/wayland-devel/2020-September/041624.html



Signed-off-by: Maciej Pijanowski <maciej.pijanowski@3mdeb.com>