Check glGetError() in rendering loop

For investigating https://github.com/iceiix/steven/issues/5
No errors seen

src/gl/mod.rs

@@ -855,6 +855,19 @@ pub fn check_framebuffer_status() {
     }
 }
 
+pub fn check_gl_error() {
+    unsafe {
+        loop {
+            let err = gl::GetError();
+            if err == gl::NO_ERROR {
+                break
+            }
+
+            println!("glGetError = {}", err);
+        }
+    }
+}
+
 impl Framebuffer {
     pub fn new() -> Framebuffer {
         let mut fb = Framebuffer(0);

src/render/mod.rs

@@ -379,6 +379,8 @@ impl Renderer {
 
         self.ui.tick(width, height);
 
+        gl::check_gl_error();
+
         self.frame_id = self.frame_id.wrapping_add(1);
     }