va: make the description pointer const

4 out of 5 plugins expect it that way and the owner should not modify
the string anyway.

modules/codec/avcodec/dxva2.c

@@ -478,7 +478,7 @@ static void Close(vlc_va_t *va)
     if (sys->hd3d9_dll)
         FreeLibrary(sys->hd3d9_dll);
 
-    free(va->description);
+    free((char *)va->description);
     free(sys);
 }
 

modules/codec/avcodec/hwdummy.c

@@ -124,7 +124,7 @@ static int Open(vlc_va_t *va, AVCodecContext *ctx, const es_format_t *fmt)
     hwctx->render = Render;
 
     va->sys = (vlc_va_sys_t *)hwctx;
-    va->description = (char *)"Dummy video decoding accelerator";
+    va->description = "Dummy video decoding accelerator";
     va->pix_fmt = AV_PIX_FMT_VDPAU;
     va->setup = Setup;
     va->get = Lock;

modules/codec/avcodec/va.h

@@ -32,7 +32,7 @@ struct vlc_va_t {
 
     vlc_va_sys_t *sys;
     module_t *module;
-    char *description;
+    const char *description;
     int pix_fmt;
 
     int  (*setup)(vlc_va_t *, void **hw, vlc_fourcc_t *output,

modules/codec/avcodec/vaapi.c

@@ -239,7 +239,7 @@ static int Open( vlc_va_t *va, int i_codec_id, int i_thread_count )
     vlc_mutex_init(&sys->lock);
 
     va->sys = sys;
-    va->description = (char *)vaQueryVendorString( sys->p_display );
+    va->description = vaQueryVendorString( sys->p_display );
     return VLC_SUCCESS;
 
 error:

modules/codec/avcodec/vda.c

@@ -115,7 +115,7 @@ static int Open( vlc_va_t *external, AVCodecContext *ctx,
     p_va->i_extradata = fmt->i_extra;
 
     external->sys = p_va;
-    external->description = (char *)"VDA";
+    external->description = "VDA";
     external->pix_fmt = PIX_FMT_VDA_VLD;
     external->setup = Setup;
     external->get = Get;

modules/hw/vdpau/avcodec.c

@@ -278,7 +278,7 @@ static int Open(vlc_va_t *va, AVCodecContext *ctx, const es_format_t *fmt)
         infos = "VDPAU";
 
     va->sys = sys;
-    va->description = (char *)infos;
+    va->description = infos;
     va->pix_fmt = AV_PIX_FMT_VDPAU;
     va->setup = Setup;
     va->get = Lock;