screen-share: use pixman_image_composite32 instead of pixman_blt

The function pixman_blt may return false in case there is no
accelerated blit function available. In this case the remote shared
screen stays black.

This has been observed on Weston compiled for aarch64. In currrent
pixman 0.38.4 there is no accelerated pixman_blt function for
aarch64 available.

Use pixman_image_composite32 instead which is guaranteed to have a
working fallback implementation.

Fixes: #253

Signed-off-by: Stefan Agner <stefan@agner.ch>