compositor: add warning if pixman_blt fails

pixman_blt may return false in case there is no accelerated blit
function available. Warn in this case.

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.

References: #253

Signed-off-by: Stefan Agner <stefan@agner.ch>
2 jobs for warn-if-blt-fails-in-screen-share in 54 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Container Prep
passed #366944
container_prep

00:00:19

 
  Build
passed #366945
build-native-meson

00:00:35