Bring back support for opencv

opencv support was disabled[1] a long time ago because ffmpeg failed
to build with it. opencv 3.x does not support C compilation mode[2]
anymore, and ffmpeg uses only C. There is no ffmpeg interest[3] in
compiling opencv in C++ mode, so opencv2 must be used here.

References
----------
[1] https://aur.archlinux.org/cgit/aur.git/commit/?h=ffmpeg-full&id=5d72306579c20bc36ff7c6ff1005b168e3b263de
[2] https://github.com/opencv/opencv/issues/10963
[3] https://trac.ffmpeg.org/ticket/7059

.SRCINFO

@@ -1,7 +1,7 @@
 pkgbase = ffmpeg-full
 	pkgdesc = Complete solution to record, convert and stream audio and video (all possible features including libfdk-aac)
 	pkgver = 6.1
-	pkgrel = 3
+	pkgrel = 4
 	url = https://www.ffmpeg.org/
 	arch = x86_64
 	license = custom: nonfree and unredistributable
@@ -84,6 +84,7 @@ pkgbase = ffmpeg-full
 	depends = onevpl-intel-gpu
 	depends = openal
 	depends = opencore-amr
+	depends = opencv2
 	depends = openh264
 	depends = openjpeg2
 	depends = openvino

PKGBUILD

@@ -3,7 +3,7 @@
 
 pkgname=ffmpeg-full
 pkgver=6.1
-pkgrel=3
+pkgrel=4
 _svt_hevc_ver='6cca5b932623d3a1953b165ae6b093ca1325ac44'
 _svt_vp9_ver='43ef8e5e96932421858762392adbbab57c84aebf'
 pkgdesc='Complete solution to record, convert and stream audio and video (all possible features including libfdk-aac)'
@@ -83,6 +83,7 @@ depends=(
     'onevpl-intel-gpu'
     'openal'
     'opencore-amr'
+    'opencv2'
     'openh264'
     'openjpeg2'
     'openvino'
@@ -224,7 +225,7 @@ build() {
         --enable-libmp3lame \
         --enable-libopencore-amrnb \
         --enable-libopencore-amrwb \
-        --disable-libopencv \
+        --enable-libopencv \
         --enable-libopenh264 \
         --enable-libopenjpeg \
         --enable-libopenmpt \