fix(generator): simplify concatenated extension construction for EGL

We don't need strcat, we already have lengths calculated, we can just
use memcpy. And this eliminates some 'strcat' usage warnings from MSVC
as well.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

Author: tycho

src/generator/c/templates/source.c.j2

@@ -305,13 +305,15 @@ static int gloam_{{ fs.spec_name }}_get_extensions_{{ api }}({{ u.ctx_arg(', ')
     /* Concatenate with a space separator. */
     concat = (char *)malloc(client_len + display_len + 2);
     if (!concat) return 0;
-    concat[0] = '\0';
-    strcat(concat, client_str);
+    memcpy(concat, client_str, client_len);
+    size_t pos = client_len;
     if (display_len) {
-        if (client_len && concat[client_len - 1] != ' ')
-            strcat(concat, " ");
-        strcat(concat, display_str);
+        if (client_len && client_str[client_len - 1] != ' ')
+            concat[pos++] = ' ';
+        memcpy(concat + pos, display_str, display_len);
+        pos += display_len;
     }
+    concat[pos] = '\0';
 
     /* Two-pass: count then fill. */
     for (j = 0; j < 2; ++j) {