lavfi/showspectrum: set default height to 512

Only power-of-two FFTs are supported; so to get a full spectrum
(that is, one up to Nyquist), the height must be a power of two.

Therefore, change the default height from 480 to 512.
Signed-off-by: Rudolf Polzer <divverent@xonotic.org>

doc/filters.texi

@@ -6019,7 +6019,7 @@ spectrum.
 The filter accepts the following named parameters:
 @table @option
 @item size, s
-Specify the video size for the output. Default value is @code{640x480}.
+Specify the video size for the output. Default value is @code{640x512}.
 @item slide
 Specify if the spectrum should slide along the window. Default value is
 @code{0}.

libavfilter/avf_showspectrum.c

@@ -51,8 +51,8 @@ typedef struct {
 #define FLAGS AV_OPT_FLAG_FILTERING_PARAM|AV_OPT_FLAG_VIDEO_PARAM
 
 static const AVOption showspectrum_options[] = {
-    { "size", "set video size", OFFSET(w), AV_OPT_TYPE_IMAGE_SIZE, {.str = "640x480"}, 0, 0, FLAGS },
-    { "s",    "set video size", OFFSET(w), AV_OPT_TYPE_IMAGE_SIZE, {.str = "640x480"}, 0, 0, FLAGS },
+    { "size", "set video size", OFFSET(w), AV_OPT_TYPE_IMAGE_SIZE, {.str = "640x512"}, 0, 0, FLAGS },
+    { "s",    "set video size", OFFSET(w), AV_OPT_TYPE_IMAGE_SIZE, {.str = "640x512"}, 0, 0, FLAGS },
     { "slide", "set sliding mode", OFFSET(sliding), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, FLAGS },
     { NULL },
 };