presets: specify the codecs.

This allows the following usages:
FFMPEG_DATADIR=presets ./ffmpeg -f lavfi -i testsrc=d=5 -vcodec libx264 -vpre ipod640 -f null -
FFMPEG_DATADIR=presets ./ffmpeg -f lavfi -i testsrc=d=5 -vpre libx264-ipod640 -f null -

The second example was broken even if documented.

presets/libvpx-1080p.ffpreset

@@ -1,3 +1,5 @@
+vcodec=libvpx
+
 g=120
 lag-in-frames=16
 deadline=good

presets/libvpx-1080p50_60.ffpreset

@@ -1,3 +1,5 @@
+vcodec=libvpx
+
 g=120
 lag-in-frames=25
 deadline=good

presets/libvpx-360p.ffpreset

@@ -1,3 +1,5 @@
+vcodec=libvpx
+
 g=120
 lag-in-frames=16
 deadline=good

presets/libvpx-720p.ffpreset

@@ -1,3 +1,5 @@
+vcodec=libvpx
+
 g=120
 lag-in-frames=16
 deadline=good

presets/libvpx-720p50_60.ffpreset

@@ -1,3 +1,5 @@
+vcodec=libvpx
+
 g=120
 lag-in-frames=25
 deadline=good

presets/libx264-ipod320.ffpreset

@@ -1,3 +1,5 @@
+vcodec=libx264
+
 vprofile=baseline
 level=13
 maxrate=768000

presets/libx264-ipod640.ffpreset

@@ -1,3 +1,5 @@
+vcodec=libx264
+
 vprofile=baseline
 level=30
 maxrate=10000000