glcolorbalance: Copy caps in transform_internal_caps()

We don't get ownership of the caps that are passed in, and doing so
causes crashes at a later time.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/546

ext/gl/gstglcolorbalance.c

@@ -181,7 +181,7 @@ static GstCaps *
 gcb_transform_internal_caps (GstGLFilter * filter,
     GstPadDirection direction, GstCaps * caps, GstCaps * filter_caps)
 {
-  GstCaps *tmp = gst_caps_make_writable (caps);
+  GstCaps *tmp = gst_caps_copy (caps);
   gint i;
   /* If we're not in passthrough mode, we can only output 2D textures,
    * but can always receive any compatible texture.