compositor: Remove unneeded left shift for ARGB/AYUV SOURCE operator

The alpha value is already in the lower 8 bits from the beginning in
this case.

Fixes gstreamer/gst-plugins-base#610
16 jobs for 1.16_backport in 3 minutes and 6 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Preparation
passed #317108
gst indent

00:00:30

passed #317107
manifest

00:00:20

 
  Build
failed #317109
build fedora x86_64

00:02:36

failed #317110
build nodebug fedora x86_64

00:02:35

 
  Test
skipped #317111
check fedora
skipped #317112
integration testsuites fedora
 
  Full Builds
skipped #317114
build cerbero android universal
skipped #317115
build cerbero cross win32
skipped #317116
build cerbero cross win64
skipped #317113
build cerbero fedora x86_64
 
  Full Tests
skipped #317120
valgrind bad
skipped #317117
valgrind base
skipped #317121
valgrind ges
skipped #317118
valgrind good
skipped #317119
valgrind ugly
 
  Apps
skipped #317122
android universal examples
 
Name Stage Failure
failed
build fedora x86_64 Build
$ cd gst-build/
$ test -d build && find build -name '*.[ao]' -delete
$ rm -rf subprojects/*/.git/
$ rm -rf build/subprojects/*/.git/
Uploading artifacts...
manifest.xml: found 1 matching files
gst-build/: found 24683 matching files
Uploading artifacts to coordinator... ok id=317109 responseStatus=201 Created token=_g2ss7Fz
ERROR: Job failed: exit code 1
failed
build nodebug fedora x86_64 Build
$ cd gst-build/
$ test -d build && find build -name '*.[ao]' -delete
$ rm -rf subprojects/*/.git/
$ rm -rf build/subprojects/*/.git/
Uploading artifacts...
manifest.xml: found 1 matching files
gst-build/: found 24683 matching files
Uploading artifacts to coordinator... ok id=317110 responseStatus=201 Created token=xYDFnoYo
ERROR: Job failed: exit code 1