Add a libplacebo-based Vulkan renderer for HDR and Vulkan video decoding support

Vulkan H.264/HEVC video decoding isn't supported with Fedora's Mesa binaries due to
patent issues, so only the VAAPI/DRM-PRIME import path was tested locally with ANV.

HDR video is getting tonemapped to my SDR monitor reasonably, so HDR output probably
works with GameScope but I haven't confirmed yet.

See #1117

app/streaming/video/ffmpeg-renderers/plvk_c.c

@@ -0,0 +1,14 @@
+// This compilation unit contains the implementations of libplacebo header-only libraries.
+// These must be compiled as C code, so they cannot be placed inside plvk.cpp.
+
+#define PL_LIBAV_IMPLEMENTATION 1
+#include <libplacebo/utils/libav.h>
+
+// Provide a dummy implementation of av_stream_get_side_data() to avoid having to link with libavformat
+uint8_t *av_stream_get_side_data(const AVStream *stream, enum AVPacketSideDataType type, size_t *size)
+{
+    (void)stream;
+    (void)type;
+    (void)size;
+    return NULL;
+}