jpeg, png, vpx: use gst_element_class_set_static_metadata()

Avoids some string copies.
diff --git a/ext/jpeg/gstjpegdec.c b/ext/jpeg/gstjpegdec.c
index 29e26b3..1579c5e 100644
--- a/ext/jpeg/gstjpegdec.c
+++ b/ext/jpeg/gstjpegdec.c
@@ -164,7 +164,7 @@
       gst_static_pad_template_get (&gst_jpeg_dec_src_pad_template));
   gst_element_class_add_pad_template (element_class,
       gst_static_pad_template_get (&gst_jpeg_dec_sink_pad_template));
-  gst_element_class_set_metadata (element_class, "JPEG image decoder",
+  gst_element_class_set_static_metadata (element_class, "JPEG image decoder",
       "Codec/Decoder/Image",
       "Decode images from JPEG format", "Wim Taymans <wim@fluendo.com>");
 
diff --git a/ext/jpeg/gstjpegenc.c b/ext/jpeg/gstjpegenc.c
index 389b0a5..a5d3c89 100644
--- a/ext/jpeg/gstjpegenc.c
+++ b/ext/jpeg/gstjpegenc.c
@@ -152,7 +152,7 @@
       gst_static_pad_template_get (&gst_jpegenc_sink_pad_template));
   gst_element_class_add_pad_template (element_class,
       gst_static_pad_template_get (&gst_jpegenc_src_pad_template));
-  gst_element_class_set_metadata (element_class, "JPEG image encoder",
+  gst_element_class_set_static_metadata (element_class, "JPEG image encoder",
       "Codec/Encoder/Image",
       "Encode images in JPEG format", "Wim Taymans <wim.taymans@tvd.be>");
 
diff --git a/ext/libpng/gstpngdec.c b/ext/libpng/gstpngdec.c
index 03e40dd..2655c43 100644
--- a/ext/libpng/gstpngdec.c
+++ b/ext/libpng/gstpngdec.c
@@ -79,7 +79,7 @@
       gst_static_pad_template_get (&gst_pngdec_src_pad_template));
   gst_element_class_add_pad_template (element_class,
       gst_static_pad_template_get (&gst_pngdec_sink_pad_template));
-  gst_element_class_set_metadata (element_class, "PNG image decoder",
+  gst_element_class_set_static_metadata (element_class, "PNG image decoder",
       "Codec/Decoder/Image",
       "Decode a png video frame to a raw image",
       "Wim Taymans <wim@fluendo.com>");
diff --git a/ext/libpng/gstpngenc.c b/ext/libpng/gstpngenc.c
index db2748c..54ca7d3 100644
--- a/ext/libpng/gstpngenc.c
+++ b/ext/libpng/gstpngenc.c
@@ -131,7 +131,7 @@
       (element_class, gst_static_pad_template_get (&pngenc_sink_template));
   gst_element_class_add_pad_template
       (element_class, gst_static_pad_template_get (&pngenc_src_template));
-  gst_element_class_set_metadata (element_class, "PNG image encoder",
+  gst_element_class_set_static_metadata (element_class, "PNG image encoder",
       "Codec/Encoder/Image",
       "Encode a video frame to a .png image",
       "Jeremy SIMON <jsimon13@yahoo.fr>");
diff --git a/ext/vpx/gstvp8dec.c b/ext/vpx/gstvp8dec.c
index b03802a..9ec1771 100644
--- a/ext/vpx/gstvp8dec.c
+++ b/ext/vpx/gstvp8dec.c
@@ -176,7 +176,7 @@
   gst_element_class_add_pad_template (element_class,
       gst_static_pad_template_get (&gst_vp8_dec_sink_template));
 
-  gst_element_class_set_metadata (element_class,
+  gst_element_class_set_static_metadata (element_class,
       "On2 VP8 Decoder",
       "Codec/Decoder/Video",
       "Decode VP8 video streams", "David Schleef <ds@entropywave.com>, "
diff --git a/ext/vpx/gstvp8enc.c b/ext/vpx/gstvp8enc.c
index 1f46163..f645e5a 100644
--- a/ext/vpx/gstvp8enc.c
+++ b/ext/vpx/gstvp8enc.c
@@ -416,7 +416,7 @@
   gst_element_class_add_pad_template (element_class,
       gst_static_pad_template_get (&gst_vp8_enc_sink_template));
 
-  gst_element_class_set_metadata (element_class,
+  gst_element_class_set_static_metadata (element_class,
       "On2 VP8 Encoder",
       "Codec/Encoder/Video",
       "Encode VP8 video streams", "David Schleef <ds@entropywave.com>, "