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>