configure: add --enable-libvpl option

This allows user to build FFmpeg against Intel oneVPL. oneVPL 2.6
is the required minimum version when building Intel oneVPL code.

It will fail to run configure script if both libmfx and libvpl are
enabled.

It is recommended to use oneVPL for new work, even for currently available
hardwares [1]

Note the preferred child device type is d3d11va for libvpl on Windows.
The commands below will use d3d11va if d3d11va is available on Windows.
$ ffmpeg -hwaccel qsv -c:v h264_qsv ...
$ ffmpeg -qsv_device 0 -hwaccel qsv -c:v h264_qsv ...
$ ffmpeg -init_hw_device qsv=qsv:hw_any -hwaccel qsv -c:v h264_qsv ...
$ ffmpeg -init_hw_device qsv=qsv:hw_any,child_device=0 -hwaccel qsv -c:v h264_qsv ...

User may use child_device_type option to specify child device type to
dxva2 or derive a qsv device from a dxva2 device
$ ffmpeg -init_hw_device qsv=qsv:hw_any,child_device=0,child_device_type=dxva2 -hwaccel qsv -c:v h264_qsv ...
$ ffmpeg -init_hw_device dxva2=d3d9:0 -init_hw_device qsv=qsv@d3d9 -hwaccel qsv -c:v h264_qsv ...

[1] https://www.intel.com/content/www/us/en/develop/documentation/upgrading-from-msdk-to-onevpl/top.html

configure

@@ -341,6 +341,7 @@ External library support:
   --disable-ffnvcodec      disable dynamically linked Nvidia code [autodetect]
   --enable-libdrm          enable DRM code (Linux) [no]
   --enable-libmfx          enable Intel MediaSDK (AKA Quick Sync Video) code via libmfx [no]
+  --enable-libvpl          enable Intel oneVPL code via libvpl if libmfx is not used [no]
   --enable-libnpp          enable Nvidia Performance Primitives-based code [no]
   --enable-mmal            enable Broadcom Multi-Media Abstraction Layer (Raspberry Pi) via MMAL [no]
   --disable-nvdec          disable Nvidia video decoding acceleration (via hwaccel) [autodetect]
@@ -1921,6 +1922,7 @@ HWACCEL_LIBRARY_NONFREE_LIST="
 HWACCEL_LIBRARY_LIST="
     $HWACCEL_LIBRARY_NONFREE_LIST
     libmfx
+    libvpl
     mmal
     omx
     opencl
@@ -6577,23 +6579,34 @@ enabled libjxl            && require_pkg_config libjxl "libjxl >= 0.7.0" jxl/dec
 enabled libklvanc         && require libklvanc libklvanc/vanc.h klvanc_context_create -lklvanc
 enabled libkvazaar        && require_pkg_config libkvazaar "kvazaar >= 0.8.1" kvazaar.h kvz_api_get
 enabled liblensfun        && require_pkg_config liblensfun lensfun lensfun.h lf_db_new
+
+if enabled libmfx && enabled libvpl; then
+   die "ERROR: can not use libmfx and libvpl together"
 # While it may appear that require is being used as a pkg-config
 # fallback for libmfx, it is actually being used to detect a different
 # installation route altogether.  If libmfx is installed via the Intel
 # Media SDK or Intel Media Server Studio, these don't come with
 # pkg-config support.  Instead, users should make sure that the build
 # can find the libraries and headers through other means.
-enabled libmfx            && { { check_pkg_config libmfx "libmfx >= 1.28 libmfx < 2.0" "mfxvideo.h" MFXInit ||
+elif enabled libmfx; then
+    { check_pkg_config libmfx "libmfx >= 1.28 libmfx < 2.0" "mfxvideo.h" MFXInit ||
 # Some old versions of libmfx have the following settings in libmfx.pc:
 #   includedir=/usr/include
 #   Cflags: -I${includedir}
 # So add -I${includedir}/mfx to CFLAGS
-                                 { check_pkg_config libmfx "libmfx >= 1.28 libmfx < 2.0" "mfx/mfxvideo.h" MFXInit && add_cflags -I${libmfx_incdir}/mfx; } ||
-                                 { require libmfx "mfxvideo.h mfxdefs.h" MFXInit "-llibmfx $advapi32_extralibs" &&
-                                   { test_cpp_condition mfxdefs.h "MFX_VERSION >= 1028 && MFX_VERSION < 2000" || die "ERROR: libmfx version must be >= 1.28 and < 2.0"; }  &&
-                                     warn "using libmfx without pkg-config"; } } &&
-                               warn "build FFmpeg against libmfx 1.x, obsolete features of libmfx such as OPAQUE memory,\n"\
-                                    "multi-frame encode, user plugins and LA_EXT rate control mode are enabled"; }
+      { check_pkg_config libmfx "libmfx >= 1.28 libmfx < 2.0" "mfx/mfxvideo.h" MFXInit && add_cflags -I${libmfx_incdir}/mfx; } ||
+      { require libmfx "mfxvideo.h mfxdefs.h" MFXInit "-llibmfx $advapi32_extralibs" &&
+        { test_cpp_condition mfxdefs.h "MFX_VERSION >= 1028 && MFX_VERSION < 2000" || die "ERROR: libmfx version must be >= 1.28 and < 2.0"; }  &&
+        warn "using libmfx without pkg-config"; } } &&
+    warn "build FFmpeg against libmfx 1.x, obsolete features of libmfx such as OPAQUE memory,\n"\
+         "multi-frame encode, user plugins and LA_EXT rate control mode are enabled"
+elif enabled libvpl; then
+# Consider pkg-config only. The name of libmfx is still passed to check_pkg_config function for --enable-libvpl option
+# because QSV has dependency on libmfx, we can use the same dependency if using libmfx in this check. The package name
+# is extracted from "vpl >= 2.6"
+    check_pkg_config libmfx "vpl >= 2.6" "mfxvideo.h mfxdispatcher.h" MFXLoad || \
+            die "ERROR: libvpl >= 2.6 not found"
+fi
 
 if enabled libmfx; then
    check_cc MFX_CODEC_VP9 "mfxdefs.h mfxstructures.h" "MFX_CODEC_VP9"