Add support for potential alternative cast command

It's easy enough to add support for screencasting tools other than FFmpeg.
An obvious example is the avconv command from libav, but almost anything
will work.  This is possible thanks to KISS, especially the stupid bit.

Also, we don't split the command line before getopts.  That was dumb.

ffcast.bash

@@ -4,10 +4,11 @@ set -e
 shopt -s extglob
 
 readonly progname=ffcast progver='@VERSION@'
-readonly cast_cmd=ffmpeg
-declare -a opt_args
-declare -a cast_args cast_cmdline x11grab_opts
-declare -a cast_args_default=(-v 1 -r 25 -- -vcodec libx264 "$progname-$(date +%Y%m%d-%H%M%S).mkv")
+readonly cast_cmd_pattern='@(ffmpeg)'
+declare -a cast_args x11grab_opts
+declare -a cast_cmdline=(ffmpeg -v 1 -r 25 -- -vcodec libx264 \
+    "$progname-$(date +%Y%m%d-%H%M%S).mkv")
+declare -- cast_cmd=${cast_cmdline[0]}
 declare -i borderless=1 mod16=0 print_geometry_only=0 verbosity=0
 PS4='debug: command: '  # for set -x
 x='+x'; [[ $- == *x* ]] && x='-x'  # save set -x
@@ -182,37 +183,18 @@ xwininfo_get_corners() {
 }
 
 #---
-# Split command line arguments into two parts:
-#
-#   ffcast ... ffmpeg ... [--] ...
-
-while (( $# )); do
-    [[ $1 == "$cast_cmd" ]] && break
-    opt_args+=("$1")
-    shift
-done
-
-if shift; then
-    cast_cmdline=("$cast_cmd" "$@")
-else
-    cast_cmdline=("$cast_cmd" "${cast_args_default[@]}")
-fi
-
-#---
-# Process arguments passed to ffcast, get a region geometry.
+# Process arguments passed to ffcast
 
 declare rootw=0 rooth=0 _x=0 _y=0 x_=0 y_=0 w=0 h=0
 IFS='x' read rootw rooth <<< "$(LC_ALL=C xwininfo -root | xwininfo_get_dimensions)"
 # Note: this is safe because xwininfo_get_dimensions ensures that its output is
 # either {int}x{int} or empty, a random string like "rootw" is impossible.
-if ! (( rootw && rooth ))
+if ! (( rootw && rooth )); then
     # %d is OK even if rootw and rooth are empty, in which case they're valued 0.
     error 'invalid root window dimensions: %dx%d' "$rootw" "$rooth"
     exit 1
 fi
 
-set -- "${opt_args[@]}"
-
 usage() {
     cat <<EOF
 $progname $progver
@@ -299,8 +281,15 @@ while getopts 'bhmpqsvw' opt; do
 done
 shift $(( OPTIND -1 ))
 
-# The rest are geometry specifications
+#---
+# Process region geometry
+
 while (( $# )); do
+    if [[ $1 == $cast_cmd_pattern ]]; then
+        cast_cmd=$1
+        break
+    fi
+    # The rest are geometry specifications
     corners_list[i++]=$(parse_geospec_get_corners "$1")
     debug "corners: %s" "${corners_list[-1]}"
     shift
@@ -324,7 +313,7 @@ if ! (( h = rooth - _y - y_ )) || (( h < 0 )); then
 fi
 
 if (( print_geometry_only )); then
-    printf "%dx%d+%d+%d" $w $h $_x $_y
+    printf "%dx%d+%d+%d\n" $w $h $_x $_y
     exit
 fi
 
@@ -354,15 +343,33 @@ if (( verbosity >= 1 )); then
     fi
 fi
 
-x11grab_opts=(-f x11grab -s "${w}x${h}" -i "${DISPLAY}+${_x},${_y}")
+#---
+# Validate cast command, set up x11grab options
+
+if shift; then
+    cast_cmdline=("$cast_cmd" "$@")
+fi
+
+# Important validation
+case $cast_cmd in
+    ffmpeg)
+        x11grab_opts=(-f x11grab -s "${w}x${h}" -i "${DISPLAY}+${_x},${_y}")
+        ;;
+    *)
+        error "invalid cast command: \`%s'" "$cast_cmd"
+        exit 1
+        ;;
+esac
+
+readonly cast_cmd
 
 #---
-# Insert x11grab options into ffmpeg command line.
+# Insert x11grab options into cast command line.
 
 set -- "${cast_cmdline[@]}"
 
 if ! shift; then
-    error 'no ffmpeg command line specified'
+    error 'no cast command line specified'
     exit 1
 fi