[visual] deconstify visual_implementation

src/media/ffmpeg.c

@@ -713,7 +713,7 @@ void ffmpeg_destroy(ncvisual* ncv){
   }
 }
 
-const ncvisual_implementation local_visual_implementation = {
+ncvisual_implementation local_visual_implementation = {
   .visual_init = ffmpeg_init,
   .visual_printbanner = ffmpeg_printbanner,
   .visual_blit = ffmpeg_blit,

src/media/none.c

@@ -8,7 +8,7 @@ printbanner(fbuf* f){
   fbuf_puts(f, "built without multimedia support" NL);
 }
 
-const ncvisual_implementation local_visual_implementation = {
+ncvisual_implementation local_visual_implementation = {
   .visual_printbanner = printbanner,
 };
 

src/media/shim.c

@@ -1,7 +1,7 @@
 #include "notcurses/direct.h"
 #include "lib/internal.h"
 
-extern const ncvisual_implementation local_visual_implementation;
+extern ncvisual_implementation local_visual_implementation;
 
 ncdirect* ncdirect_init(const char* termtype, FILE* outfp, uint64_t flags){
   visual_implementation = &local_visual_implementation;