Created alpha category and added alpha profiles from Shotcut...

Created alpha category and added alpha profiles from Shotcut https://github.com/mltframework/mlt/tree/master/presets/consumer/avformat/alpha

data/profiles.xml

@@ -10,7 +10,6 @@
    audiobitrates="256,64" defaultaudiobitrate="160"
    args="f=mp4 movflags=+faststart vcodec=libx264 progressive=1 g=15 bf=2 crf=%quality acodec=aac ab=%audiobitrate+'k'"
    speeds="preset=slower;preset=medium;preset=faster;preset=ultrafast"/>
-  <profile name="Transparent" extension="mov" args="vcodec=qtrle pix_fmt=argb"/>
   <profile name="MPEG-2" extension="mpg"
    qualities="3,15" defaultquality="5"
    audioqualities="3,7" defaultaudioquality="3"
@@ -29,6 +28,14 @@
    args="properties=x265-medium f=mp4 vcodec=libx265 acodec=aac crf=%quality ab=%audiobitrate+'k'"
    speeds="preset=slower;preset=medium;preset=faster;preset=ultrafast"/>
  </group>
+ <group name="Video with Alpha" renderer="avformat" type="av">
+  <profile name="Alpha VP8" extension="webm"
+   args="f=webm vcodec=libvpx g=120 rc_lookahead=16 quality=good speed=0 vprofile=0 qmax=51 qmin=11 slices=4 vb=2M maxrate=24M minrate=100k arnr_max_frames=7 arnr_strength=5 arnr_type=3 auto-alt-ref=0 mlt_image_format=rgb24a pix_fmt=yuva420p acodec=vorbis ab=128k"/>
+  <profile name="Alpha VP9" extension="webm"
+   args="vcodec=libvpx-vp9 vb=2M g=120 bf=2 threads=0 rc_lookahead=16 quality=good speed=3 vprofile=0 qmax=51 qmin=4 slices=4 tile-columns=6 frame-parallel=1 lag-in-frames=25 row-mt=1 auto-alt-ref=0 mlt_image_format=rgb24a pix_fmt=yuva420p acodec=libopus ar=48000 ab=128k"/>
+  <profile name="Alpha MOV" extension="mov" 
+   args="vcodec=qtrle pix_fmt=argb"/>
+ </group>
  <group name="Old-TV definition (DVD...)" renderer="avformat" type="av">
   <profile name="VOB (DVD)" extension="vob"
    qualities="5,15" defaultquality="5"