gl backend: Add line numbers to source in glsl compilation errors

Almost always the source is created by combining various sources, which
means the line numbers in the error messages are hard to use. Adding
the line numbers to the source in the error message helps with this.

gsk/gl/gskglshaderbuilder.c

@@ -47,7 +47,9 @@ check_shader_error (int     shader_id,
   int log_len;
   char *buffer;
   int code_len;
-  char *code;
+  int line;
+  char *code, *p;
+  GString *s;
 
   glGetShaderiv (shader_id, GL_COMPILE_STATUS, &status);
 
@@ -62,11 +64,29 @@ check_shader_error (int     shader_id,
   code = g_malloc0 (code_len + 1);
   glGetShaderSource (shader_id, code_len, NULL, code);
 
+  s = g_string_new ("");
+  p = code;
+  line = 1;
+  while (*p)
+    {
+      char *end = strchr (p, '\n');
+      if (end)
+        end = end + 1; /* Include newline */
+      else
+        end = p + strlen (p);
+
+      g_string_append_printf (s, "%3d| ", line++);
+      g_string_append_len (s, p, end - p);
+
+      p = end;
+    }
+
   g_set_error (error, GDK_GL_ERROR, GDK_GL_ERROR_COMPILATION_FAILED,
                "Compilation failure in shader.\nError message: %s\n\nSource code:\n%s\n\n",
                buffer,
-               code);
+               s->str);
 
+  g_string_free (s, TRUE);
   g_free (buffer);
   g_free (code);