fix(gl): use GL_NUM_EXTENSIONS constant instead of magic value

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

Author: tycho

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

@@ -181,7 +181,7 @@ static int gloam_{{ fs.spec_name }}_get_extensions_{{ api }}({{ u.ctx_arg(', ')
     if (context->GetStringi != NULL && context->GetIntegerv != NULL) {
         GLint n = 0;
         uint32_t i;
-        context->GetIntegerv(0x821D /* GL_NUM_EXTENSIONS */, &n);
+        context->GetIntegerv(GL_NUM_EXTENSIONS, &n);
         num_exts = (uint32_t)n;
         if (num_exts > 0) {
             exts = (uint64_t *)calloc(num_exts, sizeof(uint64_t));